Since f is an exponential function, it follows that there are some constants a and b such that:
[tex]f(x)=ab^x[/tex]
Therefore,
[tex]f(1)=ab=4[/tex][tex]f(2)=ab^2=12[/tex]
Hence,
[tex]\begin{gathered} \frac{ab^2}{ab}=\frac{12}{4}=3 \\ b=3 \end{gathered}[/tex]
Therefore,
[tex]\begin{gathered} 3a=4 \\ a=\frac{4}{3} \end{gathered}[/tex]
Hence, the function f is given by:
[tex]\begin{gathered} f(x)=\frac{4}{3}\times3^x \\ f(x)=4\cdot3^{x-1} \end{gathered}[/tex]
Substitute x = 3 into the function:
[tex]f(3)=4\cdot3^{3-1}=4\cdot3^2=36[/tex]