i dont understand how to do this. in my book the example is given:

x=2 or x=-1
x-2=0 or x+1=0
(x-2)(x+1)=0
x^2-x-2=0

i dont get how "(x-2)(x+1)=0 can become x^2-x-2

please explain thanks

and please can someone do this one :

write a quadratic equation in standard form for zeros: 4 and 3