Algebra Basics which involves Physics.

I really need help on this one. Find the mass and radius of three of the nine planets in our solar system. I chose pluto, venus, mars. My masses must be expressed in kilograms and the radii expressed in meters.
I am trying to calculate the gravitational acceleration on each of the three planets I selected. The masses must be measured in kilograms and the radii in meters, so the units of gravitational acceleration would turn out to be meters per squared seconds. I must now use the gravitational accelerations that I calculated in Step 2 to find the period of a 2 meter long simple pendulum on each of the nine planets. I notice the length of a simple pendulum is normally expressed in meters, so it is sufficient to replace L with the number 2 in the period expression. Are should I.
I posted this problem under Algebra and got a short answer in which I am still not clear. I was told I should have post my question under physics.

This was the answer I received, but I still dont understand totally. Can this be broken down some more. Please....

By Newtons Gravitation Law,
g = GM/r2, where
g = acceleration due to gravity (m/s2)
G = Newton's Gravitation Constant = 6.673*10-11 m3 kg-1 s-2
M = Mass of planet
r = Radius of planet

We get the following data and calculate g for each planet:

Planet Mass              Diameter    Radius               g = GM/r2
Pluto 1.25*1022 kg           2390 km     1195*103 m 0.584 m/s2
Venus 4.8685*1024 kg        12,103.7 km  6051.85*103 m 8.870 m/s2
Mars 6.4185*1023 kg 6,804.9 km    3402.45*103 m 3.700 m/s2