Try completing the square: t^2 - 2t + 1 - 1 + 4 becomes (t - 1)^2 + 3. If we set this equal to zero we get (t - 1)^2 = -3. Normally we'd take the square root of both sides, but because that -3 is negative, this is not possible, and the square root of positive 3 is not integer. Thus, the correct response here is "prime."
