If they're perpendicular, that means that angle ECM = 90 degrees and angle ACE = 90 degrees. To figure out if this works, plug that into the two equations they give:
ECM = 3x + 15
becomes
90 = 3x + 15
75 = 3x
25 = x
ACE = 4x - 10
becomes
90 = 4x - 10
100 = 4x
25 = x
Since you get x = 25 for both, you know that they are perpendicular because they both measure 90 degrees
