Multithreaded search for solutions to an inequality
You are asynchronously calculating intermediate results that are not worth parallelizing. Instead of calculating \$2^k\$ and \$3^k\$ sequentially, you do those calculations in parallel. But then you have to wait for both of them to complete before proceeding. I would expect that the overhead of setting up the async calls would negate any performance benefit.

Suppose you had a dozen friends helping you. Would you ask one of them to go off and calculate \$2^6\$, the second friend to calculate \$3^6\$, and report the results back to you so that you can check whether 6 is a solution? No, that would be crazy. You would be better off asking the first friend to check whether 6 is a solution, the second friend to check 7, the third friend to try 8, etc.  Then each of them has a chance to do some substantial work independently instead of waiting for each other's results.

Moreover, calculating \$3^k\$ would be a trivial multiplication if you already knew \$3^{k-1}\$. You should be able to avoid calling pow() altogether if you are trying increasing \$k\$.