Question

If FH =21 , and GH =18, calculate the length of FG

Answer 1

42 if you mean f×g
f is 7×h is 3=21
g is 6× h is 3=21
7×6=42

Answer 2

If the alphabets here are in sequence on the line segment then:
FH = FG + GH
21 = FG + 18
FG = 21 - 18
FG = 3

In short, Your Answer would be 3 units

Hope this helps!