Well if the room is square then you ned to find the square root of the area.
Find 
The answer to the question
The mid point is 15 because it is in the middle of 10 and 20
Take any 2 points through which the line on graph passes,
(0,-1) =(x₁,y₁)
& (2,-4)=(x₂,y₂)
slope=(y₂-y₁)/(x₂-x₁)
=[-4-(-1)]/(2-0)
=-3/2