If 1 is a root of x^2 + px + 3 = 0, then 1 + p + 3 = 0, so p = -4. The equation is x^2 - 4x + 3 = 0. Factoring gives (x - 1)(x - 3) = 0. The roots are 1 and 3.
If x = 1 is a root of the equation x^2 + px + 3 = 0, we substitute 1 for x to get 1 + p + 3 = 0, meaning p = -4. The sum of the roots is -p/1 = 4. Since one root is 1, the other root is found by subtracting it from the sum, which gives 3.