Answer:

Here's a picture of the area measured by
       :

Better find out where the area goes from positive to negative:

The measurement
       
of the triangular area on the left is negative because it is below the -axis:

Now look at the triangle on the right:

The measurement
       
of the triangular area on the right is positive:

Consequently,  
       
is given by:

Mathematica can calculate
       
directly:

Got it, and it wasn't very hard.