Answer:
x = 13
Explanation:
are valid as opposite angles, therefore equal, we solve with an equation
4x + 18 = 7x - 21
18 + 21 = 7x - 4x
39 = 3x
x = 39 : 3
x = 13
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check
4 x 13 + 18 = 7 x 13 - 21
70 = 70
the answer is good
x = 13