Answer: x = 30 (choice 3)
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To get this answer, you set the two expressions equal to one another, then you isolate x. Recall that vertical angles are congruent.
x+90 = 4x
x+90-x = 4x-x .... subtract x from both sides
90 = 3x
3x = 90
3x/3 = 90/3 ... divide both sides by 3
x = 30
Now let's check the answer
x+90 = 4x
30+90 = 4*30 ... replace every x with 30
120 = 120
The last equation is true so the answer is confirmed.