The form [tex]y=ab^x[/tex] has two unknowns, namely [tex]a[/tex] and [tex]b[/tex], so you would need to have two points to find the equation (which you do). Once you have your two points, you can create a system of equations by plugging in the x and y coordinates like below:
[tex]\left \{ {{10=ab^0} \atop {5120=ab^9}} \right.[/tex]
One thing you may remember is that ANY number to the power of 0 equals 1. Knowing that, we can replace [tex]b^0[/tex] in the first equation with 1. Since 1 multiplied by any number is just that same number, that leaves [tex]a[/tex] alone in the first equation.
[tex]\left \{ {{10=a} \atop {5120=ab^9}} \right.[/tex]
The first equation now tells us that [tex]a[/tex] is 10, so we can plug 10 in for [tex]a[/tex] in the other equation.
[tex]5120=10b^9[/tex]
Now we can divide both sides by 10 to isolate [tex]b^9[/tex].
[tex]512=b^9[/tex]
To get [tex]b[/tex] alone, we have to take the 9th root of both sides.
[tex]\sqrt[9]{512} =\sqrt[9]{b^9}[/tex]
That gives us:
[tex]2=b[/tex]
We can now plug [tex]a[/tex] and [tex]b[/tex] into the form they gave us to get:
[tex]y=10*2^x[/tex]
