[(12х - 3)-(6х - 15)] / 6 = 4- х

[ 12х - 3 - 6х + 15 ] / 6 = 4-х

(6х + 12) / 6 = 4 - х // умножить на 6

6х + 12 = 24 - 6х

6х + 6х = 24 - 12

12х = 12

х = 1