The angle on the same side of the trapezoid's leg are supplementary. It means that they are going to add up to 180°.
55 + 11x + 15 = 180
70 + 11x = 180
11x = 110
x = 10
Substitute the value of x to the equation
m∠M = 11x + 15
m∠M = 11(10) + 15
m∠M = 110 + 15
m∠M = 125
The value of x is 10
The m∠M is equal to 125°
