The convolution theorem for the Fourier Transform is:
$$y(t) = h(t)*x(t) leftarrow rightarrow Y(jw) = H(jw)X(jw)$$
The given $x(t)$ is $x(t)=cos^3(t)$. No $h(t)$ was given.
I don’t really know how to use the convolution theorem to get to the Fourier Series. Can anyone help me?