Let's say this is the point c, and that's x equals c. This is x equals d right over here, so what if we wanted to calculate this area that I am shading in right over here? You might say, well, does this actually work? Well, let's think about now what the integral, let's think about what the integral from c to d of f of x dx represents. Well, that would represent this area right over here, and what would the integral from c to d of g of x dx represent? Well, you might say it is this area right over here, but remember, over this interval, g of x is below the x-axis, so this would give you a negative value, but if you wanted this total area, what you could do is take this blue area, which is positive, and then subtract this negative area, and so then you would get the entire positive area. Well, this just amounted to, this is equivalent to the integral from c to d of f of x, of f of x minus g of x again, minus g of x. Let me make it clear, we've got parentheses there, and then we have our dx.