Question 174460
This is how I was taught: 

3x + 15 |                   4x + 12

Set the equation to 0.

3x + 15 = 0 |             4x + 12 = 0
    -15  -15 |                -12  -12
Subtract by 15. |        Subtract by 12

3x = -15 |                4x = -12
/3   /3 |                 /4    /4
Divide by 3 |           Divide by 4

x=-5 |                    x=-3

I could be wrong, but I'm pretty sure it's right.