Question

If mn=5 and m-3n=8, what is m^2n-3mn^2?

Answer 1

$m^2n-3mn^2=mn(m-3n)=5\cdot 8=40$

Answer 2

= (m^2)n -3m(n^2).  Factorise.

=mn( m-3n) :              Recall mn = 5.  (m-3n) =8.

Substituting:
=mn( m-3n)
= 5*(8) = 40.

Cheers.