Well if the room is square then you ned to find the square root of the area.
Find 
Substitute each value of x in y=2x-3
so,
first box (-2)
y= 2(-2)-3
y= -4-3
y= -7
second box (0)
y= 2(0)-3
y= 0-3
y= -3
third box (3)
y=2(3)-3
y=6-3
y=3
The microwave would cost $63.60 to buy
The answer is $7846.15 since $9.99 times 785.4 cubic inches would be 7846.146 but since we cant get a thousandth of a cent we round it to $7846.15