Dark Matter, calculating missing mass

[cherioslover]

Homework Statement

The andromeda galaxy has a velocity of 250 km/s as it rotates. The observed mass is 1 x 10^42 kg. If the galaxy is roughly a circle with a diameter of 66,00 light years how much mass is not accounted for visibly?

Homework Equations

Fc=mv^2/r and Fg=Gm1m2/r^2

The Attempt at a Solution

There was no answer, i know I am supposed to calculate the the velocity and its mass, then but to get the percent of mass missing i think you divide the difference between the the calculated mass and given mass by the calculated mass or something like that. But i am still rather confused at what to do. I am not looking for someone to complete my homework, just to help me understand how to do this question.

 

[mfb]

But i am still rather confused at what to do.

Start with the part you described - calculate the mass with the formulas. It will be larger than 10⁴² kg, and this number is (visible mass)+(invisible mass). Can you calculate the invisible mass?