In the equation f(x)=a(b^x), the term a gives us the y-intercept. This is because the y-intercept occurs when x=0. If x=0 in this equation, b^x equals 1 because any number to the power of 0 equals 1. This is then multiplied by a. Thus, is x is zero, f(x)=a.  On the graph, when x is 0, y is 3. This means that a=3.