Another physics problem - 10 points ASAP!!?

Question

Suppose that a binary star system consists of two stars of equal mass. They are observed to be separated by 360 million km and to take 5.0 years to orbit around a point midway between them. What is the mass of each? (in kg).

I actually have the answer already. It should be 5.5e29 but I don't know how to get to that. I realize a lot of other people on yahoo have posted this question but each person gives a different way of solving it so I have no clue which one is correct. Some say use kepler's 3rd law, some say use Newton's. so I know it should be one of those (preferably the easier) 

Ill note that I'm in algebra based college physics. And I've seen a lot of people answering this question using solar mass and AU? Or whatever. Well I have no idea what that is. haha and it's not part of our lesson so I'd like to avoid that. If anyone can help me with this problem ASAP I'd appreciate it! Test Wednesday! :/

ReB94 · Accepted Answer

half of the distance between the two stars will be the radius of the circle = 1.8 x 10^11 m
m1 = m2  = m

T = 5 years = 1.57 x 10^8 s

linear speed  = 2 pie r / T     [r = 1.8 x 10^11]
speed = 7.2 x 10^3 m/s

the centripetal force will be provided by the gravitational force of attraction between the two stars

(mv^2) / r = Gm^2 / (2r)^2
v^2 = Gm / 4r
m = (v^2 * 4r) / G
  m = 55.95 x 10^28 or 5.59 x 10^29 kg

Another physics problem - 10 points ASAP!!?

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