Hello from MrBillDoesMath!
Answer:
5 m^4 n
Discussion:
From the binomial theorem (or Pascal's triangle)
(m+n) ^ 5=
m^5 + 5 m^4 n + (5*4)/2! * m^3 n^2 +
5*4*3/ 3! * m^2 n^3 + 5*4*3*2/4! m n^4 + 5!/5! n^5 =
m^5 + 5 m^4 n + 10 m^3 n^2 + 10 m^2 n^3 + 5 m n^4 + n^5
The second term is shown above in bold
Thank you,
MrB