The angles given are opposite each other and are known as "vertical angles." Vertical Angles are congruent meaning that they have the same measure. So, we can just set the two expressions equal to each other and solve for x, like this:
[tex]4x+50=150[/tex]
Subtract 50:
[tex]4x=100[/tex]
Divide by 4:
[tex]x = \frac{100}{4} = 25[/tex]
Thus, x = 25°.