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The perimeter of the square is given
P = 72 in
The perimeter formula of a square is P = 4 x sides
To find the value of the side, we divide by 4
72 = 4s

4 and 4 cancels out
18 in = side
The area formula of a square is 
A = (side)²
 = (18)²
 = 324 in²