You are expected to remember and make use of the fact that adjacent angles in a parallelogram are supplementary. That means ...
(2x -10) +(5x +15) = 180
7x +5 = 180 . . . . . . . . . . . collect terms
7x = 175 . . . . . . . . . . . . . . add -5. To find x, divide by 7.
x = 25 . . . . . . . . the answer to the question
