6. 3x + 3 = 2x + 12
a. 9
b. 6
c. 3

  a.  9

Step-by-step explanation:

Observe that 2x is the x-term with the smallest coefficient, and that x-terms are found on both sides of the equal sign. Further, observe that constants are also found on both sides of the equal sign, and the constant on the side opposite 2x is 3.

To solve this, you don't want either of those terms, 2x or 3, where they are. Subtract that sum from both sides of the equation:

  3x +3 -(2x +3) = 2x +12 -(2x +3)

  x = 9 . . . . . . collect terms