Answer
$a^2+7a+10$
Work Step by Step
Replacing $x$ with $a+5$ in the given function, $
f(x)=x^2-3x
$, then $f(a+5)$ is equal to
\begin{array}{l}
(a+5)^2-3(a+5)
\\\\=
a^2+2(a)(5)+(5)^2-3a-15
\\\\=
a^2+10a+25-3a-15
\\\\=
a^2+(10a-3a)+(25-15)
\\\\=
a^2+7a+10
.\end{array}
