How to use System.Threading.dll and System.Threading.Task.dll together?

Hi everyone, I have an Unity Porject which uses MathNetNumerics.dll and therefore I also have to use the System.Threading.dll file which are storred in the Pluggins folder of the project. Now I tried to integrate another project in my project (by importing the unity asset file) and this is using System.Threading.Task (Altough I dont find SystemThreading.Tasks.dll in the pluggins folder of the project). This seems to be a problem because everytime i try to build the whole project it gives me following errors:

Assets\Scripts\ARUWPVideo.cs(190,41): error CS1983: The return type of an async method must be void, Task or Task T

Assets\Scripts\ARUWPVideo.cs(190,41): error CS0433: The type 'Task' exists in both 'System.Threading.Tasks, Version=4.0.10.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' and 'System.Threading, Version=1.0.2856.102, Culture=neutral, PublicKeyToken=31bf3856ad364e35'

How can I specify, that Tasks in the ARUWPVideo.cs file should use System.Threading.Tasks and ignore System.Threading? I dont quite understand how these file interact with each other. Also tried to set different .NET versions in the player settings. and also to delete the System.Threading.dll but this produces other errors (missing assembly reference).

The two projects are working absolutely fine on their own but when used together, it produces above errors. I also found out that updating the MathNet.Numerics could resolve the issue. But how should i do that exactly, because its a dll file? Hope for your help and thanks!