I am trying to solve this equation

f(x)=3x+1, g(x)=x^2-2x-6

I am trying to find H of g(3)

This appears to be a composite function. But what does 'H of f(3)' mean?

Slight mistake galactus! That's 'H of g(3)'! Perhaps spoiled30 meant g of g(3)? Please, can you justify that spoiled30?