Well, the easiest way to do this is simply by using the formula. The area of a rectangle is given by the formula
AREA= LENGTHXWIDTH
The reason why i'm using the formula for the area of a rectangle is because a room is usually the shape of a rectangle. Anyway;
The length is 4
The width is 3
So that is basically
AREA=4 x 3
AREA= 12.
So the area is 12n²
Hope i helped. Have a nice day