Question 316418
Length is 3 more than width. That is {{{ L = 3 + W }}}. This equation expresses the length in terms of W. 

The perimeter of a rectangle is the sum of the lengths of the four sides. A rectangle is a shape with the opposite sides having the same length, so two of the sides have length L and two of them have length W. Thus the perimeter is  {{{ P = 2L + 2W}}}. But we know L in terms of W, so we can substitute that here to get 
{{{P = 2(3+W) +2W }}}

Distributing the 2 gives 

{{{P = 6+2W+2W }}}

{{{ P = 6 + 4W }}} which is the equation for the perimeter in terms of the width W.