Explanation:
a polynomial of degree n (that means the highest exponent of x is n) must have n zeroes.
in our case that means 3 zeroes.
the best way to find them is to have the factorial form of the polynomial : to see how several terms are multiplied to give us the polynomial.
the first f(x) gives us exactly that (the second f(x) is just the form with all the multiplications done) :
f(x) = x(x + 6)(x - 2)
when is the product of factors 0 ?
only when at least one factor is 0.
so, let's see,
when is x 0 ? well, x = 0.
when is (x + 6) = 0 ? x = -6.
when is (x - 2) = 0 ? x = 2.
so, the 3 zeroes are
x = -6
x = 0
x = 2