Im pretty sure that answer is four!
It would be a a 6 by 5 inch rectangle, since the height is 6in and the width is 5in. Those are what would make up the cross section.
Slope formula:
y2-y1
-------
x2-x1
48)
1-3 -2
---- = ----
-2-4 -6
So the slope of MN is 1/3