Answer :
[tex]We\ substitude\ points\ into \ formula:\\ y= ab^{x}\\\{ {{4=ab^1} \atop {12=ab^2}} \right. |Substitude\ a= \frac{4}{b} \ from\ first \ equation\\12= \frac{4}{b}*b^2\\
12=4b|:4\\
b=3\\a= \frac{4}{3} \\
y=4^x[/tex]