F= W*I/L find and expression for L

You do this the same way you would if you only had one variable. That is, if F, W and I were numbers instead of letters, what would you do? You're still attempting to isolate one variable. You're still moving things back and forth between sides, and you still have to do the opposite operation to get rid of something.

The only difference is that you can't actually perform the operation because you have no numbers, so you just have to leave it.

So if you had

and you're solving for X, you divide out the Z.



You just leave the division as letters since you can't actually do anything with it.

(As a side note, when you have fractions, you can still multiply through by the denominator, even when it throws the variable you're solving for to the other side. It'll still work out and can be the fastest way.)

Give it a shot.