You have the following inequality:
12 - 3x < 18
in order to solve the previous inequality, proceed as follow:
12 - 3x < 18 subtract 12 both sides
-3x < 18 - 12
-3x < 6 divide by -3 both sides (and change the sense of the inequality)
x > 6/(-3)
x > -2
Hence, the solution fot the given inequality is x>-2