A square has all four sides that are the same length. So all four sides are 12 cm
area = side*side = 12*12 = 144 cm^2
Or you could say
area = (side)^2 = (12)^2 = 144 cm^2
Squaring a number is the same as multiplying it by itself. Hence the name "square" refers to a side length pairing up with its corresponding area value.
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Side note: The perimeter is 4*(side) = 4*12 = 48 cm. It's possible that choice B is put in there as a trick answer.
