Consider the function x^2+x-4
Estimate the area between the graph and the x-axis between x=2 and x=4 using four rectangles and right end points.
4 rectangles means 0.5 interval width.
(Assuming they are all the same width)
f(0.5) = -3.25
f(1.0) = -2.00
f(1.5) = -0.25
f(2.0) = +2.00
So, the area is 0.5(-3.25-2.00-0.25+2.00) = -1.75
The area is negative since it's mostly below the x-axis.