C is a positive constant. How would you calculate the Fourier Transform of $\frac{sin^2(3\pi C t) }{ 3 \pi^2 C t^2 }$? As it is not easy to calculate the Fourier integral $ \int_{-\infty}^{\infty} \frac{sin^2(3\pi C t) }{ 3 \pi^2 C t^2 } \cdot e^{-j 2 \pi f t} dt $ or look up in a Fourier table.
$\operatorname{sinc}(x) = \frac{\sin(\pi x)}{\pi x}~.$
$\frac{sin^2(3\pi C t) }{ 3 \pi^2 C t^2 } = 3C sinc(3Ct)^2$
By using the signal processing definition of the Fourier transform, it should be something like this according the WolframAlpha.
