.
I answered this question twice at this forum, but it comes again and again.
Actually, this problem is intended for those who wants to solve it on his (or her) own,
and for such a person my hints would be enough.
But since it comes again and again, it makes it clear to me that this visitor EITHER is not able to use my hints,
OR has different goals than to exercise his (or her) mind.
Therefore, I will not explain it more.
I simply give you the reference to relevant page in the Internet
https://www.quora.com/How-do-I-prove-that-no-terms-in-the-arithmetic-progression-2-5-8-will-be-a-perfect-square