Figure A shows level curves of

Figure A shows level curves of $z=f(x,y)$.
(a) Estimate the direction and length of grad $f$ at $P,Q,R$.
(b) Locate two points where grad $f$ is parallel to $\mathbf{i}+\mathbf{j}$.
(c) Where is $|\text{grad }f|$ largest? Where is it smallest?
(d) What is your estimate of $z_{\max }$ on this figure?
(e) On the straight line from $P$ to $R$, describe $z$ and estimate its maximum.

Gilbert Strang, Calculus, 3rd edition (Wellesley-Cambridge Press; MIT OpenCourseWare RES.18-001) · §13.4 Directional Derivatives and Gradients, #33