x² -16=0
1) Since we have two roots and any quadratic equation can be rewritten as once we have the roots:
So let's find one equation whose roots are 4 and -4, with coefficient a=1
(x-4)(x+4)=0
x² -16=0
2) So the equation x² -16= 0 has roots x_1=4 and x_2= -4 S={-4,4}