Chapter 7 Exponential and Logarithmic Functions - 7.7 Write and Apply Exponential and Power Functions - 7.7 Exercises - Quiz for Lessons 7.6-7.7 - Page 536: 12

$y=\frac{5}{3}\cdot3^x$

Let $y=ab^x$. Then our equations are: $15=ab^2$ and $45=ab^3$. If we divide the second equation by the first one we get: $3=b$. Then $15=a\cdot3^2=9a\\a=\frac{5}{3}$. Thus $y=\frac{5}{3}\cdot3^x$