Question 830603
A perimeter is a fence.  It requires 2 pieces of length and 2 pieces of width.  (2L + 2W = P)
You're given the following information:
2L + 2W = 24
L = 3W
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Replace the value of L in the original equation.
2(3W) + 2W = 24
Multiply
6W + 2W = 24
Add the Ws
8W = 24
Divide each side by 8
W = 3
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Now that you have the width, you can calculate the Length (you were given that length is 3x the width). 
L = 3*3, or 9
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Let's plug those values in to make certain.
(2*9) + (2*3) = 24
18 + 6 does indeed equal 24.