As you have already written, the exponential function have the following form:
[tex]\begin{gathered} y=B(C)^x \\ B,C\in\mathfrak{\Re } \end{gathered}[/tex]With the given points
[tex]\lbrace(0,12),(1,3)\rbrace[/tex]We can make a substitution in the general form and find the coefficients "B" and "C".
Making the substitution:
[tex]y(0)=12\Rightarrow B(C)^0=12\Rightarrow B=12[/tex][tex]y(1)=3\Rightarrow12(C)^1=3\Rightarrow C=\frac{3}{12}=\frac{1}{4}=0.25[/tex]With this, we have the values for our coefficients, and we can make the substitution in the general form:
[tex]\begin{gathered} y=B(C)^x \\ \begin{cases}B=12 \\ C=0.25\end{cases}\Rightarrow y=12(0.25)^x \end{gathered}[/tex]