What is the smallest real number in the domain of the function g(x) = sqrt((x - 3)^2 - (x - 18)^2)?
The quantity inside square root must be non-negative for $g(x)$ to be a real-valued function. This constraint $(x - 3)^2 - (x - 18)^2 \ge 0$ can be solved for $x$ to $x\ge 10.5$, so 10.5 is the desired minimum.
