GFI = GI arc ==》 GI arc = x + 42
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GHI = GI arc ÷ 2
==》 x - 9 = ( x + 42 ) ÷ 2
Multiply both sides by 2
==》2 × ( x - 9 ) = 2 × ( x + 42 / 2)
==》 2x - 18 = x + 42
Add both sides 18
==》 2x - 18 + 18 = x + 42 + 18
==》 2x = x + 60
Subtract both sides x
==》 2x - x = x - x + 60
==》 x = 60
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GHI = x - 9
GHI = 60 - 9
GHI = 51