(m²-6mn+9n²)÷(m-3n)-(4m²-9n²)÷(2m-3n)
=(m-3n)²÷(m-3n)-(2m-3n)(2m+3n)÷(2m-3n)
=(m-3n)-(2m+3n)
=m-3n-2m-3n
=-m-6n