look at the patterns:
(5)(5)(5) = 5^3
here, your exponent is 3. meaning that 5 has been multiplied three times.
(-10)(-10)(-10)(-10)(-10) = (-10)^5
here, your exponent is 5. meaning that (-10) has been multiplied five times.
(a)(a) = (a)^2
here, your exponent is 2. meaning that (a) has been multiplied twice.
@amistre64 showed you something with k instead of a. these letters are just place holders, meaning they could be any value. it is the same concept, the k has been multiplied by how many times it appeared in the equation. he used n as a placeholder for the exponents to indicate that n will be how many times k will be multiplied.