You need to know this cold: if r is a root of polynomial p(x), then x-r divides p(x) evenly (i.e. no remainder) AND p(r) = 0.
These facts will be used countless times from Algebra on up the mathematics subjects.
Example: find the roots of ---> p(x)/(x-2) = x-7 (no remainder)
x=7 is one root
x=2 is the 2nd root (the total count of roots always equals the highest power found in p(x), here we have two roots because the highest power of x in p(x) is 2)
p(7) = 7^2-9(7)+14 = 49-63+14 = 0 <<< p(7) = 0 as expected
check p(2) on your own.