29 − x = 18, x = 29 − 18 = 11.
x + 3 = 79, x = 79 − 3 = 76.
12 : x = 3, x = 12 : 3 = 4.
x · 9 = 90, x = 90 : 9 = 10.
Ответ: x = 11; x = 76; x = 4; x = 10.