It is easiest to simply just substitute any of the choices into the original equation and see which one satisfies the equation.
Given the equation: 3 |5 + x| = 12
Using choice A. x = 7
3 |5 + 7| = 12
36 =/= 12
Using choice B. x = 1
3 |5 + 1| = 12
18 =/= 12
Using choice C. x = -9
3 |5 + -9| = 12
12 = 12
Therefore, the correct answer is C. -9
