Error CS0120 and CS1612 - Wheelcollider sidewaysFriction and Rigidbody.velocity

I'm currently working on a car project due in 2 days, and having some issues:

The error I'm having is:

"CS1612(XX,25(XX represents line 59-62)): Cannot modify a type return value of 'UnityEngine.WheelCollider.sidewaysFriction'. Consider storing the value in a temporary variable"

Here's my (C#)Code (I Used the '*' to show the error area):

 using UnityEngine;
 using System.Collections;
 
 public class DriveScriptC : MonoBehaviour{
 
     public float motorForce;
     public float SteerAngle;
     public WheelCollider BLWheel;
     public WheelCollider BRWheel;
     public WheelCollider FLWheel;
     public WheelCollider FRWheel;
     public float brakeForce;
 
     
 
     public GameObject fin;
 
     public bool CheckpointA;
 
     public float slip;//slip of the velicle
 
     public float hit;
 
     public WheelFrictionCurve sidewaysFriction;
 
     // Use this for initialization
     void Start () 
     {//what vars are
         slip = 0;
         CheckpointA = false;
         fin = GameObject.FindWithTag ("Finish");
     }
     
     // Update is called once per frame
     void Update () {
         float v = Input.GetAxis ("Vertical") * motorForce;//
         float h = Input.GetAxis ("Horizontal") * SteerAngle;//declaring var 
         //motor power
         BLWheel.motorTorque = v;
         BRWheel.motorTorque = v;
         //steering
         FLWheel.steerAngle = h;
         FRWheel.steerAngle = h;
 
 
         if (Input.GetKey (KeyCode.Space)) {//When SpaceBar is down(held), apply brake
             BLWheel.brakeTorque = brakeForce;
             BRWheel.brakeTorque = brakeForce;
         }
 
         if (Input.GetKeyUp (KeyCode.Space)){//When SpaceBar is up(not held), don't apply brake
             BLWheel.brakeTorque = 0;
             BRWheel.brakeTorque = 0;
         }
 
 
         *slip = 0.001 + 0.022 / (rigidbody.velocity.magnitude + 1);*
         *//velocity-slip curve*
         *FLWheel.sidewaysFriction.stiffness = slip*Time.deltaTime; //         X*
         *FRWheel.sidewaysFriction.stiffness = slip*Time.deltaTime; //       XX*
         *BLWheel.sidewaysFriction.stiffness = slip*Time.deltaTime; //    XXX*
         *BRWheel.sidewaysFriction.stiffness = slip*Time.deltaTime; //XXXX*
 
         WheelHit FLhit;
         WheelHit FRhit;
         WheelHit BLhit;
         WheelHit BRhit;
 
         if(FLWheel.GetGroundHit(out FLhit)){
             if(FLhit.collider.gameObject.tag == "Finish"){
                 if(CheckpointA == true){//if Checkpoint selected
                     print("FINISH!");//Print "Finish"
                 }
             }
         }
         if(FRWheel.GetGroundHit(out FRhit)){
             if(FRhit.collider.gameObject.tag == "Finish"){
                 if(CheckpointA == true){//if Checkpoint selected
                     print("FINISH!");//Print "Finish"
                 }
             }
         }
         if(BLWheel.GetGroundHit(out BLhit)){
             if(BLhit.collider.gameObject.tag == "Finish"){
                 if(CheckpointA == true){//if Checkpoint selected
                     print("FINISH!");//Print "Finish"
                 }
             }
         }
         if(BRWheel.GetGroundHit(out BRhit)){
             if(BRhit.collider.gameObject.tag == "Finish"){
                 if(CheckpointA == true){//if Checkpoint selected
                     print("FINISH!");//Print "Finish"
                 }
             }
         }
     }
 }

I do admit, I am kind of a newbie...