Х(2х+3)-5(х²-3х)=3Х(7-Х)
2x^2 + 3x+5x^2 + 15x=21x +3x^2 -6x^2 + 3x+15x=21x -6x^2 + 18x-21x=0 -3x(2x+1)=0 x=0, 2x=-1 x=-1/2