Модуль дает два значения: положительное и отрицательное.
1) |9x - 18| = 27
9x1 - 18 = -27; 9x1 = 18 - 27 = -9; x1 = -1
9x2 - 18 = 27; 9x2 = 18 + 27 = 45; x2 = 5
2) |15x - 10| = 5
15x1 - 10 = -5; 15x1 = 10 - 5 = 5; x1 = 5/15 = 1/3
15x2 - 10 = 5; 15x2 = 10 + 5 = 15; x2 = 1
3) |9x + 15| = 6
9x1 + 15 = -6; 9x1 = -15 - 6 = -21; x1 = -21/9 = -7/3
9x2 + 15 = 6; 9x2 = -15 + 6 = -9; x2 = -1