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。
If you can't even figure out this math problem, it must be super hard.
この場合、次のような言い方ができますよ。
ーIf you can't even figure out this math problem, it must be super hard.
「おまえでもこの数学の問題が解けないなら、めちゃくちゃ難しいんだろうな」
to figure out this math problem で「この数学の問題を解く」
ご参考まで！
アメリカ合衆国 
カナダ 
