Solve the equation w=Cr^-2 for r.

Is this right?

w/C=r^-2

r=sqrt(C/w)

And if so, I still seem to be having troubles answering the next part.

2. Suppose that an object is 100 pounds when it is at sea level. Find the value of C that makes the equation true. (Sea level is 3,963 miles from the center of the Earth.)

I got that C=1570536900

If all this is correct then how do i do the next part.

3. Use the value of C you found in the previous question to determine how much the object would weigh in Death Valley (282 feet below sea level). (hint: 1 mile=5280 feet)

Lost completely... please help...