The figure below shows some level curves of a differentiable function $f(x,y)$.

Based only on the information in the figure, estimate the directional derivative:
$f_{\vec u}(3,3) \mbox{ where } \vec u= (-\boldsymbol{i}-\boldsymbol{j})/\sqrt{2}$

$f_{\vec u}(3,3) \approx$

(Hint: to obtain the best estimate, find the average rate of change over equal increments in the $\vec u$ and $-\vec u$ directions, then find the mean of these two estimates.)