Answer:
m² + 6mn + 9n²
Explanation:
given
(m + 3n)²
= (m + 3n)(m + 3n)
each term in the second factor is multiplied by each term in the first factor, that is
m(m + 3n) + 3n(m + 3n) ← distribute parenthesis
= m² + 3mn + 3mn + 9n² ← collect like terms
= m² + 6mn + 9n²