mass of a star

[A_I_]

Suppose there is a planetary system in which a planet with an average distance of 6 AU from the star has an orbital period of 3 years. What is the mass of the star?

The answer should be in SOLAR MASSES.


I tried to use the formula: P^2 = (4pi^2*a^3)/G(m+M)
but it didnt work:(

any hint?
thanks

[Tide]

This is a simple variation problem and you should recognize that

\frac {M P^2}{R^3}

is a constant so you can easily setup the appropriate proportion.