Question

If EG= 10 and EF=4, then FG=

Answer 1

If EFG form a triangle, then we need more information (like an angle) 
If F is a point on line segment EG, then 
FG = EG − EF = 10 − 4 = 6

Answer 2

If EF is a segment of EG then you would subtract EF from EG to get 6