Angles FGC and FHD are equal by the Corresponding Angles Theorem.
Make them equal to each other in an equation.
3x - 9 = 2x + 1
Add 9 to both sides.
3x = 2x + 10
Subtract 2x from both sides.
x = 10
Plug in 10 for x in the expression for angle FGC, 3x - 9.
3(10) - 9
30 - 9 = 21
∠FGC = 21°.