Question 931516
Find logarithms of both sides of the equation form.


{{{ln(y)=ln(ab^x)}}}, choosing base e just for convenience.
{{{ln(y)=ln(a)+ln(b^x)}}}
{{{ln(y)=ln(a)+x*ln(b)}}}
{{{highlight_green(ln(y)=x*ln(b)+ln(a))}}}


That is a linear equation with slope ln(b) and vertical axis intercept ln(a).


Treat your given points properly.  
The point given (1,4) must be converted into (1,ln(4)).
The other point given (2,12) must be used as (2,ln(12)).
The slope will be {{{ln(b)=(ln(12)-ln(4))/(2-1)}}}.  Find the value for b using this slope information.


How to find ln(a):
Solve the linear form for ln(a) and then choose either of the converted points.
{{{ln(a)=ln(y)-x*ln(b)}}}, and you would know ln(b) from the previous calculations....
Find the value for a.