The answer key: With every equation, for the first set of integers, use subtraction, ie 9-4=5; for the second set of integers, use addition, ie 9+4=13. You combine both sets of integers for the "correct" answer. The answer for this first equation would be 513. For the last one, the formula would be 15-3=12; and 15+3=18, combined together is 1218.