Skill check: Partial derivatives and the gradient

Let $f(x, y) = x^2 - xy$ and $\vec{\text{v}} = \left[\begin{array}{c} 2 \\ -1 \end{array} \right]$. Compute the directional derivative of $f$ along $\vec{\text{v}}$. $\nabla_\vec{\text{v}} f(x, y) = $ [[☃ expression 1]]