4x/(4x+3y)+3/(1+3y)=[4x(1+3y)]/[(4x+3y)(1+3y)]+[3(4x+3y)]/[(4x+3y)(1+3y)]=[4x(1+3y)+3(4x+3y)]/[(4x+3y)(1+3y)]=(4x+12xy+12x+9y)/(4x+12xy+3y+9y²)=(16x+12xy+9y)/(4x+12xy+3y+9y²)