perimeter = length + length + width + width
if the perimeter is 42 then:
42 = length + length + width + width
if the length is 6 times the width, then
6 x width = length
42 = (6 x width) + (6 x width) + width + width
(i'm just going to start using w to represent width to make it easier
42 = 6w + 6w + w + w
combine all the w's
42 = 14w
divide both sides by 14
w = 3
the width is 3