so, first you have to find the area of the floor and carpet alone. so for the carpet, it would be
12 x 10 = 120 ft (for the floor)
and
7 x 5 = 35 ft (for the carpet)
now, to find the area of the floor that isn't covered by the rug, you subtract them.
120 - 35 = 85 ft 85 ft is your answer