Gravitational Acceleration given mass and radius

[EliteCodexer]

Homework Statement

If you lived on a planet with five times the mass of Earth and twice the radius, what would be the gravitational acceleration at the surface of your planet?

Homework Equations

GM/r^2
Mass of Earth = 6.00*10^24 kg
Radius of Earth = 6.38*10^3 km

The Attempt at a Solution

So, when I started out doing this, I quickly plugged in Earth's radius (in m), mass (in kg) and the gravitational constant just to make sure it came out at 9.8 because I know that's what Earth's gravitational acceleration from the surface is, but none of the answer I get are even close. I can't even get this right and I still have to do the actual problem (using 5x the mass and 2x the radius).

 

[SteamKing]

Homework Statement

If you lived on a planet with five times the mass of Earth and twice the radius, what would be the gravitational acceleration at the surface of your planet?

Homework Equations

GM/r^2
Mass of Earth = 6.00*10^24 kg
Radius of Earth = 6.38*10^3 km

The Attempt at a Solution

So, when I started out doing this, I quickly plugged in Earth's radius (in m), mass (in kg) and the gravitational constant just to make sure it came out at 9.8 because I know that's what Earth's gravitational acceleration from the surface is, but none of the answer I get are even close. I can't even get this right and I still have to do the actual problem (using 5x the mass and 2x the radius).

Instead of making us guess where you went wrong, how about posting your work?

 

[BiGyElLoWhAt]

it does work. we did it the other day sitting around.
try
GM/r^2
Mass of Earth ≈ 5.97*10^24 kg
Radius of Earth ≈ 6.38*10^3 KM = 6.38*10^6 m
G≈ 6.67 × 10^-11

\frac{M_{earth}}{r_{earth}^2}G ≈ \frac{5.97*10^{24}}{6.38*10^{12}}*6.67*10^{-11}
=\frac{5.97*6.67*10}{6.38} ≈9.805

 

[haruspex]

I can't even get this right and I still have to do the actual problem (using 5x the mass and 2x the radius).

It's worth persevering to get that right, but you don't need it for this question.
You can solve it just by taking g = 9.8m/s² for Earth and adjusting it. What effect will 5 times the mass have on that? What effect will twice the radius have?