We know he definitely bought 3 books for $12 each so we immediately put that in the equation. Then we add that to the unknown (6 books times however much he bought them for) and then we equal it to 84 (his budget). Now we do 3 x 12 which is 36. Our new equation is 36 + 6x = 84. 84 - 36 = 48 so we know 6x = 48. Now divide 48 by 6 to get rid of the 6 and we learn that the variable equals 8. So, x = 8. We now know that he bought 6 books at 8 dollars each.
