How do you calculate a^5 x a^16 / a^4?

When each term has the same base, add the indices that are multiplied. Then subtract the indices that are divided.

a^5 x a^16 / a^ 4 = a^(5+16) / a^4 

                            = a^ 21 / a^4

                            = a^(21-4)

                            = a^17