Question

In the solution of the equation 5 - 3x = 2x + 9, 3x is added to the equation first. Which of the following should be done next?

Answer 1

The answer should be "add -9"

Answer 2

Regardless of which action you choose, you should include "to both sides."
For example, you could add -9 to both sides, or you could subtract 9 from both sides (same result).  Adding 3x to both sides results in 

5 - 3x = 2x + 9
   + 3x = 3x
----------------------
5 = 5x + 9

We need to solve for x.  The best way to do this is to subtract 9 from both sides, resulting in -4 = 5x, so that x = -4/5 (final answer).