The area of a rectangle is obtained by multiplying the lengths of its sides. In this case, each wall is 10 feet by 12 feet. So, the area of one wall is 10 * 12 square feet which is 120 square feet. There are three walls that need to be painted. The total area that needs to be painted is obtained by multiplying the area of one wall by the total number of walls. So, total area is 120 * 3 = 360 square feet. It is stated that 1 can of paint can cover 420 square feet and that Meredith has 1 can of paint with her. So, Meredith has enough paint to paint the three walls in her room.