a) 15 + x = 28
x = 28 - 15
x = 13
б) x - 9 = 0
x = 9
в) x - 23 = 15
x = 15 + 23
x = 38
г) x - 17 = -2
x = -2 + 17
x = 15
д) x + 11 = -9
x = -9 - 11
x = -20
e) x - 17 = -31
x = -31 + 17
x = -14
ж) x + 11 = 0
x = -11
з) x - 16 = -16
x = -16 + 16
x = 0
и) 15 - x = 16
-x = 16 - 15
-x = 1
x = -1
Ответ: а) x = 13, б) x = 9, в) x = 38, г) x = 15, д) x = -20, е) x = -14, ж) x = -11, з) x = 0, и) x = -1.