Learn Arabic (Modern Standard): الحل ليس سهلا.

In Arabic, لا is generally used to negate action verbs (like "I do not go"). Because "The solution is easy" is a nominal sentence (a sentence where "is" or "are" is implied), we must use ليس to negate it.

Why does the word for "easy" have an extra alif at the end (سهلا) instead of just being سهل?

That extra alif holds the double 'a' sound (fathatan) ending, making it read as "sahlan". This happens because ليس forces the descriptive word that comes after it (the predicate) into the "accusative" case. For an indefinite word like this, it means changing the default "un" ending to an "an" ending, which adds that extra alif.

Would ليس change if the word "solution" was a feminine noun?

Yes! Even though ليس is used to negate nominal sentences, it behaves like a verb and must match the gender of the subject. Since الحل (the solution) is masculine, we use ليس. If the subject were a feminine word, like المشكلة (the problem), we would have to use the feminine form ليست.

Why does "solution" have ال at the beginning, but "easy" doesn't?

In Arabic, the subject of an "A is B" sentence usually needs "the" (ال) to be specific ("The solution"). The describing word stays indefinite (no ال) so it completes the sentence. If both words had ال, it would just mean the phrase "the easy solution", rather than the full sentence "The solution is easy".