Since it has an initial value of 2, the value of a is 2.
Substitute this into the general equation: y = abx
Substitute the values 3 and 128 for x and y, respectively, then solve for b.
Create two equations substituting the values of the ordered pairs for x and y into the equation y = abx.
Solve the system of equations for a and b.
Be sure you use your results for a and b to write the equation.