|m+3|+n²-4n+4=0
|m+3|+(n-2)²=0
∵|m+3|≥0
(n-2)²≥0
∴|m+3|=0
(n-2)²=0
解得 m=-3
n=2
故3m²n+3mn²
=3mn(m+n)
=3×(-3)×2×(-3+2)
=18