x^2 +5x^2-7x+15x-21=9
6x^2 +8x -12 =0
2(x^2 +4x -6) =0
x1= -2+căn 10
x2= -2-căn 10
x^2+(x+3)(5x-7)=9
<=>x^2+5x^2+8x-21-9=0
<=>6x^2+8x-30=0
<=>2(3x^2+4x-15)=0
<=>3x^2+4x-15=0
<=>3x^2-5x+9x-15=0
<=>x(3x-5)+3(3x-5)=0
<=>(x+3)(3x-5)=0
<=>x+3=0 hoặc 3x-5=0
+)x+3=0<=>x=-3
+)3x-5=0<=>3x=5<=>x=5/3