Question

Find the counterexample to this statement: If ab = 3, then either a or b must be equal to 3

Answer 1

if a or b is 3, but the remaining variable is 0, then the statement is not true

Answer 2

That's not a counterexample. You're starting with the conclusion.

I'm assuming a and b are integers. Otherwise, there are an infinite number of counterexamples, which is quite obvious.

Consider a = -3, b = -1. ab = 3, but neither a nor b is equal to 3.