Hi, i really need help with this question

find the fourier series of the function f(t)= 3sin(t) + sin(4t).

I know that the function is odd (so an=0), and has period 2pi, but when i try finding bn, integrating between -pi and pi, i keep just getting 0. This cant be right, where am I going wrong?

Please help!

Katy (Happy)