The room is not in shape of a square but a rectangle.
Question says Length is: 19 inches
Diagonal is: 20.6 inches
In a square all sides are equal
Rectangle has just two sides equal
First, you would have to add d to both sides to get rid of it
x+d=ab+c/b
Then you would multiply b by both sides to get rid of the b in the denominator
x+d(b)=ab+c
After that, you would subtract c from both sides
x+d(b)-c=ab
Then you would divide both sides by a
x+d(b)-c=b
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a
