Find the exponential function f(x)=Cax\displaystyle {f{{\left({x}\right)}}}={C}{a}^{{x}} whose graph goes through the points (0,3)\displaystyle {\left({0},{3}\right)} and (2,12)\displaystyle {\left({2},{12}\right)}.
a=\displaystyle {a}=   ,
C=\displaystyle {C}=   .