-3x = 21
Explanation: on the -3x-5 side, add 5 to -5. That cancels it out so you are left with -3x. On the 16 side, add 5. 16+5 is 21. Therefore, -3x = 21 is equivalent to -3x-5 = 16.
Also:
If you are also supposed to solve the equation the answer is x = -7.