Chapter 7 Exponential and Logarithmic Functions - 7.7 Write and Apply Exponential and Power Functions - 7.7 Exercises - Skill Practice - Page 534: 15

$y=0.12\cdot x^{2.322}$

Let $y=ax^b$. Then our equations are: $3=a4^b$ and $15=a8^b$. If we divide the second equation by the first one we get: $5=2^b\\b=\ln_{2}{5}\approx2.322$. Then $3=a4^{2.322}\\a=\frac{3}{4^{2.322}}\approx0.12$. Thus $y=0.12\cdot x^{2.322}$