Explanation:
f(x) = x² - 2x - 3
since it is a quadratic there are only two zeros
f(-1) is a zero since when you substitute in -1 it will all add up to zero making it a factor of f(x)
the same works for f(3) when substituted will give 0 when added up together
factor the quadratic to get (x+1)(x-3)
set each factor to zero you will get
x1=-2 x2=3