(1) k^2 > 400
(2) k^3 < 8000
Analyzing the question:
Note we are only given that k is a two-digit integer, it might be negative.
This simplifies to |k|>20|k|>20, which is either k>20 or k <−20. Therefore k might be greater than 19 or less, insufficient.
k^3<8000, k^3<20^3, k<20.
k must be 19 at most, so it is indeed less than or equal to 19. Sufficient.