Suggestion: one more contour label in this activity

[rhinopotamus]

This activity is really fun for connecting to a visual representation of derivatives. However, it's a bit annoying that the contour over on the right, the one through (3, -1), isn't labeled.

That contour could reasonably be either at height 4 (in which case f_x(2,0) > 0) or at height 2 (in which case f_x(2,0) < 0).

[rhinopotamus]

Also on this activity, the solution manual (thanks for publishing this!!!!!) has that f_y(-1.2, -1) > 0, with which I disagree. My favorite point for which f_y is positive is (0, 2).