1) x²-9=0

x²=0+9

x²=9

x=-3 x=3

2) x²-x=0

x(x-1)=0

x=0 x-1=0

x=1

3) x²+x-6=0

x²+3x-2x-6=0

x(x+3)(x-2)=0

(x+3)(x-2)=0

x+3=0 x-2=0

x=-3 x=2

4) x²-5x+4=0

x²-x-4x+4=0

x(x-1)-4(x-1)=0

(x-1)(x-4)=0

x-1=0 x-4=0

x=1 x=4