What are the zeroes of f(x) = x2 − 9x + 20?

First, factor the polynomial. Then, set each binary answer to zero and solve. Those are the values of x where the graph either crosses or touches the graph.
x²-9x+20
(x-5)(x-4)

x-5=0 | x=5
x-4=0 | x=4

The zeros are 4 and 5