Let h be the vertical distance between the graphs of f and g in region S. Find the rate at which h changes with respect to x when x is equal to 1.8. So we have region S right over here. You can't see it that well since I drew over it. And what you see in region S, the function f is greater than the function g. It's above the function g. And so we can write h of x. We could write h of x as being equal to f of x minus g of x. And what we want to do is we want to find the rate at which h changes with respect to x. And so we could write that as h prime of, we could say h prime of x, but we want the rate when x is equal to 1.8.