Remark
There are a couple of ways you can do this. The best way is probably the way that avoids dividing by a minus sign. It is a bit longer but division can sometimes cause problems.
Solution
Add gt to both sides.
f + gt = i Subtract f from both sides.
gt = i - f Divide by g
[tex]\boxed{\text{t=}\dfrac{i - f}{g}}[/tex]
