Question

If FG = 2 units, FI = 7 units, and HI = 1 unit, what is GH?

Answer 1

we know that

$FI=FG+GH+HI$

solve for GH

$GH=FI-(FG+HI)$

we have

$FI=7\ units\\FG=2\ units\\HI=1\ units$

substitute in the formula above

$GH=7-(2+1)$

$GH=7-3$

$GH=4\ units$

therefore

the answer is

$GH=4\ units$

Answer 2

Does the line look like this?

F________G_______H_______I

2+X+1=7
3+X =7
X = 4