my bad. did not notice the FAQ ;;
It's correct. It just happened to have 2 different meaning, though I agree that it should have been listed on different rows. ON! THE! OTHER! HAND! You can check the etymology of solve and soluble and see that they're literally brothers born from the Latin Solvere "to dissolve, loosen". This logical connection between solving a problem (abstract) and dissolving something, untying something (physical) is astoundingly universal.
It is a combination of the verb 解ける + negation ない. Strictly speaking, it is not a one-word Keiyoshi as the dictionary suggests.
But see if you say 解けない問題, the combination 解けない is used as a Keiyoshi modifying 問題.
For the meaning of 解ける, see
https://jisho.org/search/%E8%A7%A3%E3%81%91%E3%82%8B
correct definition?
is it supposed to say insoluble? i feel like that's pretty different to unsolvable lol