the angles 3x and 54° are linear angles, namely they're both sitting on a straight line JN.
a straight line always have an angle of 180°, so 3x and 54° add up to 180°.
3x + 54 = 180
3x = 126
we know that ∡JML = 3x, thus ∡JML = 126°, because 3x = 126.