Assuming the measurements are also in feet, finding square feet is an easy multiplication.
(I'm going to break it down so I can do it in my head)
12*14 = 12*12 + 12*2 = 144 + 24 = 168 sq ft
400 square feet.
240 square feet
162 square feet.
525 square feet.
300 square feet.
252 square feet.
120 square feet.
16' x 15' = 240 square feet.
A simple rectangular room 20 feet by 18 feet has 360 square feet.
A 10x14 room has an area of 140 square feet (10 feet x 14 feet = 140 square feet).
There is 121 square feet in a 11'x11' room.
There are 270 square feet in a 15'x18' room.