Determine all the zeros for the function f(x)=(x^2+2x-8)(x-6)

This function is of the third degree so it must have three zeroes. One of them is x=6, so all you need to do is determine the zeroes for the quadratic part. You can factor it as (x+4)(x-2), and in this way you can easily see that the other two zeroes are x=-4 and x=2.

These are the multiples of 15 but what you are probably asking or what are the factors of 15