A5 inch tall balloon shoot doubles in height every 3 days. if the equation y=ab^x, where is x is the number of doubling periods, represents the height of the bamboo shoot, what are the values of a and b?

In this item if we let x be the number of doubling period, that is number of days divided by 3, and y be the height of the bamboo shoot, in the equation
                                     y  = ab^x
The value of a is 5 because that is the original height of the shoot and b is 2 because we are talking about the doubling rate. The equation is therefore,
                                    y = 5(2^x)