Answer
$525m^5n^2$
Work Step by Step
Using the $(r+1)$st term of the expansion of $(a+b)^n$, which is given by $\dfrac{n!}{(n-r)!r!}a^{n-r}b^r$, then the $
3
$rd term of $
(m+5n)^7
$ is
\begin{array}{l}
\dfrac{7!}{5!2!}(m)^{5}(5n)^{2}
\\\\=
21m^5(25n^2)
\\\\=
525m^5n^2
\end{array}
