I've been at this problem for a while!

Find the directional derivative of the given function at the point (3,-1) in the direction theta=(pi)/4.

f(x,y)=4x+xy^2-5y

fx=4+y^2

fy=2xy-5

applying ponts:

fx=4+(-1)^2=5

fy=2(3)(-1)-5=-11

<5, -11>

But how would I find my unit vector in the direction of theta=pi/4?