Question 533933
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You know the perimeter of a rectangle is the length of all the sides.

L+L+W+W = 2L+2W = P

P = 78

W is unknown, so call it X.

L is unknown, but it's 9 feet more than the W so call it X+9.

Plug all of these into the formula.

2(X+9) + 2X = 78   (multiply 2 by what's inside the ())

2X + 18 + 2X = 78   (combine like terms)

4X + 18 = 78  (Subtract 18 from both sides)

4X + 18 - 18 = 78 - 18 --->  4X = 60  (divide both sides by 4)

4X/4 = 60/4   ---> X = 15

W = X = 15 (width)
L = W + 9 = X + 9 = 15 + 9 = 24.

Check those in the formula to ensure the P comes to 78.

15 + 15 + 24 + 24 = 78  HURRAH!