If f(-2) = f(3) = f(6) = 0, then you know the three roots of the cubic are -2, 3 and 6, so that it's factorized form would be
f(x) = a (x + 2) (x - 3) (x - 6)
where a is a constant.
We're also given f(5) = 28, so
28 = a (5 + 2) (5 - 3) (5 - 6)
28 = -14a
a = -2
Then the cubic function is
f(x) = -2 (x + 2) (x - 3) (x - 6)
and its expanded form is
f(x) = -2x ³ + 14x ² - 72
