Find the directional derivative of $f(x,y) = \sin(x + 2y)$ at the point (-3, 1) in the direction $\theta = \pi/4$.
The gradient of $f$ is:
$\nabla f = \langle$,$\rangle$
$\nabla f(-3, 1) = \langle$,$\rangle$
The directional derivative is: