The formula for the volume of a rectangular prism is
[tex]V=l\cdot h\cdot w[/tex]You need to write the formula for w, note that the width is being miltiplied by "lh"
to cancel this multiplication you have to divide it by "lh" and to keep the equality valid, what is done to one side of the expression must be done to the other, so divide V by "lh" too
[tex]w=\frac{V}{lh}[/tex]