Struggling with Rate of Change for Rational Functions

Hey guys, I've been googling for hours on how to solve this with no luck!

I need to find the average and instantaneous rate of change of a rational function:

As you get farther from Earth’s surface, gravity has less effect on you. For this reason, you actually weigh less at higher altitudes. A person who weighs 55kg can use the function:

W(h) = 6400(55)/ (h + 64000)

to find their weight, W in kgs, at a specific height, h in feet above sea level, above the Earth’s surface.

Find the average rate of change from heights of 750 ft to 1200 ft above sea level, and find the instantaneous rate of change at 1200 ft above sea level.

I know that average rate of change is [W(1200)-W(750)]/1200-750 right? But when I plug in values for h the answers aren't even close. I know I'm missing something but I just don't understand what!

The answer posted is -0.0065 but I get like -0.00008