Finding rate of change of f at p in direction of unit vector - finding unit vector

Question

I have f(x,y) = sin(2x+3Y) and a point P(-6, 4).

I found $\nabla f= \langle2cos(2x+3y), 3cos(2x+3y)\rangle$ and $\nabla f (-6, 4) = \langle2,3\rangle$.

The final part of the question with a final answer of $\sqrt{3}-\frac{3}{2}$, was to find the rate of change of f at p in the direction of the unit vector. I think I'm supposed to use $D_uf= \nabla f*u$ but I'm not sure how to find u here.