multiply (n^3+8)(n-4) can you give me an example on how to solve this please

do things just as you would with numbers:

(2+5)(7-3)
2*7 + 5*7 - 2*3 - 5*3

You have

(n^3+8)(n-4)
n^3*n + 8*n - n^3*4 - 8*4
now simplify each term
n^4 + 8n - 4n^3 - 32
and arrange in descending powers
n^4 - 4n^3 + 8n - 32

Sometimes you may have several terms with the same power. In that case, collect them into a single term. For example,

n^3 - 2n^2 + 3 + 9n^2 - 2n + 3 - 4n
would become simply
n^3 + 7n^2 - 6n + 6