Answer:
21
Explanation:
straight line = 180 degrees ( H,D,R)
180 = 7x - 3 + 2x - 6
180 = 7x + 2x - 3 - 6 (group the same type number together)
180 = 9x -9 (simplify the numbers)
189 = 9x (+9)
21 = x (divide by 9)
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