I am suppose to find the directional derivative for the function f(x,y,z) = xy + z^2

at the point (2,3,4) in the direction of a vector making an angle of 3pi/4 with the grad f(2,3,4)

I have done several with this function with other specific directions but this one is confusing me.

I know the grad is y i + x J + 2z k so at (2,3,4) this would be 3i + 2j + 8 k but where do I go from here???? Please someone nudge or push me in the correct direction!!!! Thanks