x^2-x-20=0
if X^2 + aX + b = 0
then ;
product of roots is b
sum of roots is a
there you have to look for numbers whose sum is a and product is b
in the equation we have
x^2-x-20=0
it can be easily seen that
-5 + 4 = -1
-5 * 4 = -20
then we shall have to expand the equation
we have X^2 +4X -5X -20 = 0
X(X+4) -5(X+4) = 0 from (X^2 +4X) +(-5X -20)
then (X-5)(X+4)=0