Akitchen is 12 ft. by 14 ft. with 10 ft. high walls.

how much would it cost to put wallpaper in this room?

cost = $

The answer is 520x.

Let x be the price of wallpaper per square feet.
Dimensions of a kitchen are:
width: w = 12 ft
length: l = 14 ft
height: h = 10 ft

If you need to put wallpaper in the room, you will put it only on the walls. There are 4 walls in the kitchen: two walls which dimensions are 12 ft and 10 ft, and two walls which dimensions are 14 ft and 10 ft. Since walls are of rectangular shape, we will use the following formula for the area of walls:
A = 2 * w * h + 2 * l * h
A = 2 * 12 * 10 + 2 * 14 * 10
A = 240 + 280
A = 520 ft²

So, if 1 square foot costs x, 520 square feet will cost 520x

It is going to be the third choice