Answer:
see explanation
Explanation:
the term + m²n should be - m²n
(m² - n)(m² + 2n²)
each term in the second factor is multiplied by each term in the first factor, that is
m²(m² +2n²) - n(m² + 2n²) ← distribute the parenthesis
=
+ 2m²n² - m²n - 2n³