Chapter 5 - Exponential and Logarithmic Functions - 5.6 Logarithmic and Exponential Equations - 5.6 Assess Your Understanding - Page 311: 18

$25$

Step 1. Rewrite the equation as $log(x^2-21x)=2$ Step 2. Thus $x^2-21x=10^2=100\longrightarrow x^2-21x-100=0\longrightarrow (x-25)(x+4)=0\longrightarrow x=-4, 25$. Step 3. Check answers, only $x=25$ fits the original equation.