Fourier Transforms - The Convolution Theorem.

Ok so I've seen the convolution theorem written as:



F(h(x)[itex]\otimes[/itex]g(x))=H(k)G(k)



(And this is how it appears when I have a quick google).



My book then does a problem in which is uses:



F(h(x)g(x))=H(k)[itex]\otimes[/itex]G(k)



Where H(k)=F(h(x)) and similarly G(k)=F(g(x)),

and F represents a fourier transform




My question

- I can't see how these are equivalent at all?



Many Thanks to anyone who can help shed some light !