31. We'd use a number line to show -4 + 3 = 3+ -4 by first starting at -4 and moving 3 to the right, ending at -1, then comparing that to starting at 3 and moving 4 to the left, also ending at -1.
In general we see the order of the operands doesn't matter for addition: that's the commutative property.
32 Call the pool depth d. The equation is
4 + d = 12
so
d = 12 + -4
a sum of integers, i.e.
d = 8
