Answer
All real numbers apart from $5$ and $3$.
Work Step by Step
We know that for rational expressions the domain is all the real numbers, excluding when the denominator is $0$.
$x^2-8x+15=x^2-3x-5x+15=x(x-3)-5(x-3)=(x-3)(x-5)$
Here the denominator is $0$, when $x=5$ or $x=3$, thus the domain is all real numbers apart from $5$ and $3$.
