Finding the mass of a planet using gravitational acceleration and radius

[PrometheuSIT]

Homework Statement

Planet Z is 5000 km in diameter. The free-fall acceleration on Planet Z is 7.00 m/s^2. What is the mass of Planet Z?

Homework Equations

Well Newton's gravitational law G(M1M2)/R^2
And I cannot think of much else

The Attempt at a Solution

On one of the earlier homeworks we were given a table in which there were fictional planets and their ratios of mass and radius to Earth so I tried using

gG M(earth)/(R(earth))^2 = gG M(z)/(R(z))^2
Where the "g"s are each planet's free fall acceleration.
so that the Gravitational constant would cancel and I'd have the Mass of planet Z

For the mass of the Earth I used 5.97E34, for the radius of the Earth I used 6387.1 km
I know that the radius needs to be in meters and would be correctly notated as 6.3871E6

I don't know how to figure this one out

 

[LowlyPion]

Welcome to PF.

F = ma = GMm/r²

a = GM/r²

 

[PrometheuSIT]

Thank you very much