The point of an equation is to keep an equality, so anytime you apply something on one side of the equation, you have to make sure you apply the exact same thing on the other side.
Starting with ##v=lwh##
Your goal is to find what is ##w##
In order to solve it I will apply different division on both sides in order to keep the equality:
##v=lwh##
##v/l=(lwh)/l## See how I applied the same division on both side
##v/l=(wh)## Simple rule, I have the same thing over and under the fraction so it disapears
##v/(lh)=(wh)/h## Once again I apply the same division on both sides
And finally ##v/(lh)=w## is your final answer
