Normals for Displacement Shader

I have written up a basic displacement shader. I am using it on a basic plane, and am interpolating between two heightmaps. However i dont know how to find the normals after i have displaced them. Especially since the height they are can be displaced varies.

it all boils down to:

• float amt = tex2dlod from one texture

• float amt2 tex2dlod from the second texture

• v.vertex.xyz += v.normal lerp(amt,amt2,_DisplacementInterpolation)_DisplacementHeight;

Can anyone point me in the right direction, as to how to find out what the new normals are?