The formula for the perimeter, p, of a rectangle is p = 2l + 2w, where l is the length and w is th width sovle the formula for the perimeter of the rectangle for w.

                                 P  =  2L + 2w

Subtract  2L
from each side:          P - 2L  =  2w

Divide each side
by  2 :                        (P - 2L) / 2  =  w

You could also write it as      (P/2  -  L)  =  w
Both answers are the same.