For the function f(x)=3x2+4x\displaystyle {f{{\left({x}\right)}}}={3}{x}^{{2}}+{4}{x}, evaluate and simplify.

f(x+h)f(x)h\displaystyle \frac{{{f{{\left({x}+{h}\right)}}}-{f{{\left({x}\right)}}}}}{{h}} =