The one that is not equivalent is k^6/k^15.
When multiplying exponents, you just add them:
k^8×k = k^9
k^7×k^2=k^9
When dividing exponents, you just subtract them:
k^13/k^4=k^9
As you can see, all of these equations so far equal k^9.
The reason why k^6/k^15 is not equivalent is because:
k^6/k^15=k^-9
(6-15=-9)
Our exponent, while still being 9, is now a negative nine and therefore not equivalent.