The measure of angle MNO is 105, and the value of x is 21.
This can be figured out through the equation 75 + 5x = 180
first, you add -75 to both sides to cancel it out from the left, and are left with 5x = 105
there, we know that MNO is 105 because 105 = 5x = MNO
to figure out the value of x, then, you divide both sides by 5 to get 21.