Hey joatmon.

I'm not sure given only this amount of information, but one thing that may help is to note that if a is congruent 1 MOD 3 then a^2 is also congruent to 1 MOD 3.

So a = 3b + 1 for some b. This means

a^2 = (3b+1)^2 = (3b)^2 + 6b + 1 = 3*(3b^2 + 2b) + 1 = 3c + 1 so a^2 is congruent to 1 MOD 3.