Chapter 5 - Exponential and Logarithmic Functions - 5.6 Logarithmic and Exponential Equations - 5.6 Assess Your Understanding - Page 311: 17

$5$

Step 1. Rewrite the equation as $log(x^2+15x)=2$ Step 2. Thus $x^2+15x=10^2=100\longrightarrow x^2+15x-100=0\longrightarrow(x+20)(x-5)=0\longrightarrow x=-20,5$. Step 3. Check answers, only $x=5$ fits the original equation.