We don't know f(x). The way to do this is to work out h(f(g(x))) by replacing x in the definition of f(x) by 3x^2-4, which is the definition of g(x). Evaluate f(x) when x is replaced in this way and you end up with (f o g)(x). Now replace x in sin(x) with the expanded expression for f. That will give you (h o f o g)(x).
[You can edit your own question and insert the missing definition of f(x) and then save the amended question.]