Boundary Values & Fourier Series · Medium problem

The Fourier series of the square wave $f(x) = 1$ on $(0, \pi)$, $-1$ on $(-\pi, 0)$ is $\dfrac4\pi\sum_{k\text{ odd}}\dfrac{\sin kx}{k}$. Compute $\pi\cdot b_3$, where $b_3$ is the coefficient of $\sin3x$. Express as a common fraction.