You solve an equation like this by adding the opposite of the constant to both sides of the equation.
... V -16 +16 = -32 +16 . . . . . addition property of equality: if a=c, then a+b = c+b
... V + 0 = -16 . . . . . . . . . . . . additive inverse property of integers: -16+16 = 0
... V = -16 . . . . . . . . . . . . . . . identity element of addition: V+0 = V
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You can always do the same thing to both sides of an equation. Here, it is useful to add the opposite of -16 to both sides. That way the constant on the left becomes zero, so you only have the variable by itself—which is what you want.