x²-a*x+6*x+2*a+12=0
x²-a*x+6*x+2*a=0-12=-12
x²+x²-a+6+2*a=-12
2(x²)-a+2*a=-12-6
2(x²)-a+2*a=-18
2(x²)+2=-18
2(x²)=-18-2
2(x²)=-20
x²=-20/2
x²=-10
x=-√10
