Question 1140383
{{{h(x)=sqrt(12x^2+21)}}}


Choose {{{g(x)=12x^2+21}}}, and then {{{f(x)=sqrt(x)}}}.


h(x) may have other composed possibilities.


Another way:
g(x)=x^2 and then f(x)=sqrt(12x+21) so composing f of g would be replace x with g=x^2,   {{{sqrt(12(x^2)+21)}}} and small simplification step.