Basically, you just perform whatever operations are needed to get the variable you DON'T know by itself on one side of the equal sign, plug in the variables you do know, and you have it. In your example, say you don't know the length, but you know the others:
V = l*w*h
V/w*h = l*w*h/w*h
V/wh = l
Now l is by itself, so you can substitute values for the other variables and solve for l.