Given g(2)=81\displaystyle {g{{\left({2}\right)}}}={81}, g(2)=5\displaystyle {g}'{\left({2}\right)}={5} and f(x)=g(x)\displaystyle {f{{\left({x}\right)}}}=\sqrt{{{g{{\left({x}\right)}}}}}, find f(2)\displaystyle {f}'{\left({2}\right)}.