Math Help - Finding the area of this region

1. Finding the area of this region

I've attached a picture of the problem I'm working on. First I set each equation equal to y so I have y=x+a and y=-x+b which would make the points of intersection (0,0) and (-2,-5) so I then subtracted -x+b from x+a (substituting in the corresponding values) and got the final answer of four. Somewhere I must be going wrong could someone walk me through how to do a problem like this????