Fourier Coefficients
Note that \$\text{sinc}\,{\pi k T} = \frac{\sin{\pi kT}}{\pi k T} \$. The inverse fourier transform of this is a box. 

Note further that your frequency-domain expression is precisely \$\text{sinc}\,{\pi k T} \cdot \text{sinc}\,{\pi k T} \$. Furthermore, note that multiplication in the frequency domain is the same as convolution in the time domain; the convolution of a box with itself is a triangular pulse. This should suffice as a starting point toward solving the problem; computing the box/triangle support and normalization is left to the reader so as to avoid doing your exercise in its entirety for you.