Gravitational Pulls- Acceleration decreases as object is farther from center

[Phoenixtears]

Homework Statement

A sensitive gravimeter at a mountain observatory finds that the acceleration due to gravity is 0.0057 m/s2 less than that at sea level. What is the observatory's altitude?
___m

Homework Equations

g= (GM)/r^2

The Attempt at a Solution

Alrighty, my attempt makes sense to me, so I'm sure where I'm going wrong.

I began by finding the mass. So, I used 9.8 m/s2 as the g and then the Earth's radius as r:

9.8= (6.67E-11)M/6.37E6 (<----- the Earth's radius)

M= 9.359E17 kg

Then I subtracted the acceleration change given from 9.8:

9.8-0.0057 = 9.7943 m/s2
That is my new g.

Then, using the found mass, I plugged in my new numbers:

9.7943= (6.67E-11)(9.36E17)/ r^2
r=2523.4 m

That, however, was not the correct answer. I can't see my error. Could anyone help me out?

Thanks in advance!

~Phoenix

 

[atyy]

Do you need to square the Earth's radius?

 

[Phoenixtears]

Do you need to square the Earth's radius?

Sorry. I never came back to edit that out. My mistake. I actually realized that I was being dense and could just use the Earth's mass- 5.98E24.

Yet this is still the incorrect answer. Can anyone help me?

 

[D H]

We can't tell why your result is incorrect if you don't tell us what your result is and how you obtained it. In other words, show your work, please!