[tex]y=ab^x \\ b>0 \\ \\ (2,135) \\
x=2 \\ y=135 \\ \downarrow \\ 135=ab^2 \\ a=\frac{135}{b^2} \\ \\
(4,1215) \\
x=4 \\ y=1215 \\ \downarrow \\ 1215=ab^4 \\ a=\frac{1215}{b^4}[/tex]
[tex]\frac{135}{b^2}=\frac{1215}{b^4} \\
135b^4=1215b^2 \ \ \ |\div 135b^2, \ b>0 \\
b^2=9 \\
b=-3 \ \lor \ b=3 \\
b>0 \\
b=3 \\ \\
a=\frac{135}{b^2}=\frac{135}{3^2}=\frac{135}{9}=15 \\ \\
\boxed{y=15 \times 3^x}[/tex]
