You have to be given what a1 is.  Like, if the problem said

a1 = -3

an = - a(n-1) + n - 3

Then a2 would be - a1 + 2 - 3 = 3 + 2 - 3 = 2

and a3 would be -a2 + 3 - 3 = -2 + 3 - 3 = -2