What number should be added to 51 to get a sum - 21

We can write out a formula in order to solve this. Since sum represents addition, if x represents the unknown number, we can write the formula: x+51=-21. Then, to solve for x, you would subtract 51 from both sides to get x=-72 which is the final answer.

51 + - 72 = - 21

Adding a negative seventy-two to fifty-one will give you a sum of negative twenty-one