What we are looking at here is a comparison with -12 being the smallest value, -3x - 15 being the second smallest value, and 36 being the largest value in the sequence. What you need to do is to make the problem into two separate problems:
-12 < -3x - 15
and
-3x - 15 < -36
Then we solve the equations like so (using PEMDAS):
-12 < -3x - 15
+15 +15
3 < -3x
/-3 /-3
-1 > x (the sign was changed the other way because when you divide by a negative it has to be flipped)
