If Sachiko lets x represent the length of the side of the square and she wants to find the length of the perimeter of the square, it is appropriate for her to ...
... B. Set the area equal to x², solve for x, and then multiply the value of x by 4.
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The area of a square of side length x is x·x = x². The perimeter of a square of side length x is x+x+x+x = 4·x.