a<0

y = a(x+2) (x-3) (x-½)

 = a(x²-x-6) (x-½)

 = a(x³-1.5x²-5.5x+3)

e.g.,

y = -2x³ + 3x² + 11x - 6

Here is one solution:

f(x) = −(x+2) (2x−1) (x−3)

This has leading coefficient = −2

There are infinitely many possible functions for f, all of the form:

f(x) = −k(x+2) (2x−1) (x−3)

where k is any real positive number

Here is a graph of 3 such function:

https://www.desmos.com/calculator/kfdmp0ndcx