We want to determine the length of the room, which is a rectangle. The width and the area of the room are given. Therefore, we will use the formula for area of a rectangle.
A=l w
Look closely at the expression for the area, x^2-100. It can be expressed as the difference of two squares.
Recall the formula to factor a difference of squares.
a^2- b^2 ⇔ ( a+ b)( a- b)
We can apply this formula to our expression.
x^2- 10^2 ⇔ ( x+ 10)( x- 10)
Since we know that the width of the room is x-10 feet, its length must be equal to x+10 feet. Therefore, the correct option is B.
