Write an expression (or equation) that represents the number of square feet of wallpaper you will need if the height of the family room is x feet, with a
length and width that are each 3 times the height of the room. The family
room has 1 door, which is 3 feet wide and 7 feet tall.

Given

room height is x feet

room length is 3x feet

room width is 3x feet

a door 3 ft wide by 7 ft tall

Find

The net area of the wall, excluding the door

Solution

The area of the wall, including the door, is the room perimeter multiplied by the height of the room. The room perimeter is the sum of the lengths of the four walls.

... gross wall area = (3x +3x +3x +3x)·x = 12x²

The area of the door is the product of its height and width.

... door area = (7 t)×(3 ft) = 21 ft²

Then the net wall area, exclusive of the door is ...

... net wall area = gross wall area - door area

... net wall area = 12x² -21 . . . . square feet

look a the sides they are perpendicular

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