Gravitation equations and expressions - gravitational field strength

[dory]

Homework Statement

i have 2 questions on my holiday homework that I am not 100% sure on they are:

Homework Equations

1) write down an algebraic expression for g at a distance r from the centre of the planet. The planet can be treated as a point mass of magnitude M as its centre

AND

2) write down a formula for the gravitational field stength g at the surface of the planet of mass M

The Attempt at a Solution

is one of them g=Gm/r^2
i am so confused

 

[mfb]

is one of them g=Gm/r^2

If you replace m with M, yes.