Rectangular Photograph Word Problem

A rectangular photograph is twice as long as it is wide. The perimeter of the photograph is 84 cm.
Find the length and the width of the photograph.

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x = width
2x = length   {twice as long as it is wide} 

Perimeter of a rectangle is 2(width) + 2(length) 

2x + 2(2x) = 84   {perimeter is 2(width) + 2(length)}
2x + 4x = 84   {multiplied}
6x = 84   {combined like terms}
x = 14   {divided each side by 6}
2x = 28   {substituted 14, in for x, into 2x} 

width = 14 cm
length = 28 cm

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