I need help solving this problem, im not sure what to do with the $\displaystyle \theta = \frac{2\pi}{4}$. The formula is $\displaystyle D_u f(p) = u *\nabla f(p) $Quote:

If $\displaystyle f(x,y)=-4x^2-1y^2$, find the value of the directional derivative at the point $\displaystyle (-1,-3)$ in the direction given by the angle $\displaystyle \theta = \frac{2\pi}{4}$.

$\displaystyle f_x(x,y) = -8x, f_y(x,y) = -2y $