Finding Period with Double Stars

Homework Statement

A certain double star consists of two identical stars, each of mass 3.0E30 kg, separated by a distance of 2.0E11 m between their centres. How long does it take to complete one cycle? Give your answer in seconds.

Homework Equations

Gm / 4π^2 = r^3/ T^2 (I'm pretty sure the motion will be elliptical)

The Attempt at a Solution

I tried the above equation, and isolated for T, but I got the wrong answer. The answer is 7.9E7 seconds.