Answer:
x = 10
Step-by-step explanation:
Angles 3 and 7 are corresponding angles. They are congruent when lines 'a' and 'b' are parallel.
∠3 = ∠7
80 -x = 90 -2x
x = 10 . . . . . . . . . . . . add 2x-80 to both sides
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Check
When x=10, both angles 3 and 7 are 70°, so lines 'a' and 'b' are parallel.
