Question 829921
That's a pretty smart way to do it.  A perimeter is simply 2L + 2W, or what you need to build a fence around the shape.
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What should he do next?  Measure the length of the string.
And then repeat the process.  Measure twice, cut once.
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He can also double-check the measurement of the string by measuring the length and width of the painting and adding twice the length and twice the width together.
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If he's done it right, the numbers should match.