Word problem involving missing side of triangle

In a certain triangle, the longest side is 1 foot longer than the second longest side, and the second-longest side is 1 foot longer than the shortest side.  If the perimeter is 21 feet, how many feet long is the shortest side?

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x = shortest side
x + 1 = second-longest side   {second-longest side is 1 ft longer than shortest side}
x + 2 = longest side   {longest-side is 1 ft longer than second-longest side} 

Perimeter is the distance around the outside of an object.

x + x + 1 + x + 2 = 21   {added up all sides and set equal to 21}
3x + 3 = 21   {combined like terms}
3x = 18   {subtracted 3 from each side}
x = 6   {divided each side by 3}

6 ft is the shortest side

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