We are told that l is the length of the rectangle. We are told that the widht is 4 less than half the lenght. So, first, we calculate the half of l, which would be
[tex]\frac{l}{2}[/tex]the part that says "4 less than" means that we should subtract 4 from the half of length. So we get
[tex]\frac{l}{2}-4[/tex]now, this expression should be equal to the width , that we'll note as w. So we end up with the equation
[tex]w=\frac{l}{2}-4[/tex]