If you can't solve this problem, then this one is seriously too difficult.
If this problem is unsolvable even for you, then this is too difficult, no joke.
ご質問ありがとうございます。
① "If you can't solve this problem,"=「君にこの問題が解けなければ、」
"then this one is seriously too difficult."=「この問題は難しすぎるだろう。」
② "If this problem is unsolvable even for you,"=「もし君でもこの問題が解けないならば、」
"then this is too difficult, no joke."=「これは難しすぎる、マジで。」
ご参考までに。