12: Using substitution you should have calculated
Now make your conclusion.
Comparison can be based on the fact that exponential grows faster than powers, which is true even if the exponential is slowed down by putting a square root in the exponent. However, then it is no longer a part of the scale of powers and thus it should be proved. To make things easier (and shorter), try proving that the denominator in the given terms is (for large k) larger than k2.