3x(5y-4)-y(4-5y)+(15y-12)-3xy(4-5y)
= 15xy - 12x - 4y + 5y^2 + 15y - 12 - 12xy + 15xy
= 30xy - 12x - 4y + 5y^2 + 15y - 12
= 30xy - 12x + 11y - 5y^2 - 12
3x(5y-4)-y(4-5y)+(15y-12)-3xy(4-5y)
= 15xy - 12x - 4y + 5y^2 + 15y - 12 - 12xy + 15xy
= 30xy - 12x - 4y + 5y^2 + 15y - 12
= 30xy - 12x + 11y - 5y^2 - 12