Chapter 7 Exponential and Logarithmic Functions - 7.7 Write and Apply Exponential and Power Functions - 7.7 Exercises - Skill Practice - Page 534: 19

$y=0.569\cdot x^{1.907}$

Let $y=ax^b$. Then our equations are: $8=a4^b$ and $30=a8^b$. If we divide the second equation by the first one, we get: $3.75=2^b\\b=\ln_{2}{3.75}\approx1.907$. Then $8=a4^{1.907}\\a=\frac{8}{4^{1.907}}\approx0.569$. Thus $y=0.569\cdot x^{1.907}$