Write a polynomial function of least degree with integral coefficients that has the given zeros 2,-1,-5

Since it has zeros at x = 2, -1, and  -5, then by definition, (x - 2), (x + 1), and (x + 5) are factors of the polynomial.

A standard polynomial is in the form:


Since R(x) = 0, then

and you can just expand that out to get a cubic polynomial, as expected.