I'm guessing the diagram shows EF and FG are on the same straight line and that EF = FG. You really need a diagram if this is the wrong assumption.
FG = 4n - 3
FG = 4(6) - 3
FG = 24 - 3
FG = 21 <<<< answer.
n + 6 = 4n - 3 Add 3
n + 6 + 3 = 4n
n + 9 = 4n Subtract n
9 = 4n - n
9 = 3n Divide by 3
n = 3.
But you want n + 6
6 + 3 = 9
FG = 9 <<<< answer.
