Ex 3.7
13. find the area enclosed between the lines y=4 and y=1-x in the second quadrant

4.5 because when you draw this on a coordinate plain you are able to see a right triangle whose base and height is equal to 3.  The formula of a trigangle is 1/2(base*heigth) and the values of our base is 3 and the height is 3, so once you plug in those values you should get a value of 4.5.