Help with A2 physics??

A lunar landing module and its parent craft are orbiting the moon at a height above the surface of 6.0X10^5 m. The mass of the luner module is 2.7X10^3 kg. The graph below shows the variation of the gravitational force acting of the module with height above the moon's surface. The radius of the moon is 1.4X10^6 m.

{a graph showing force against height above surface - when height above surface= 6X10^5 force= 2.5X10^3}

1) using the data from above, find the gravitational force acting on the lunar module and hence, find its speed when the orbital height is 6.0X105
The furthest I got with this one was dividing force (2.5X10^3) by the mass (2.7X10^3) to find the gravitational field strength, not even sure if that's right...


2) Using the data from above, find the change in the gravitational potential energy of the lunar module as it descends from its orbit to the surface of the moon
I'm almost sure that there's an easy formula to use for this one, but again, not sure how to go about it.

thanks anyways, I would really appriciate a bit help as it looks like a quite easy question that I should be able to do.
thanks ^-^