a) (2 + x) : 5 = 6
2 + x = 6 $\times$ 5
2 + x = 30
x = 30 - 2
x = 28
b) 2 + x : 5 = 6
x : 5 = 6 - 2
x : 5 = 4
x = 4 $\times$ 5
x = 20
c) 2x - 3 = 11
2x = 11 + 3
2x = 14
x = 14 : 2
x = 7
d) 3 . (x - 18) + 75 = 0
3 . (x - 18) = -75
x - 18 = -75 : 3
x - 18 = -25
x = -25 + 18
x = -7