1) x^2 - (x+1)(x+3)
= x^2 - (x^2 + 3x + x + 3)
= x^2 - (x^2 + 4x + 3)
= x^2 - x^2 - 4x - 3
= -4x - 3

2) x^2 - (x-1)(x-3)
= x^2 - (x^2 - 3x - x + 3)
= x^2 - (x^2 - 4x + 3)
= x^2 - x^2 + 4x - 3
= 4x - 3

3) x^2 - (x+1)(x-3)
= x^2 - (x^2 - 3x + x - 3)
= x^2 - (x^2 - 2x - 3)
= x^2 - x^2 + 2x + 3
= 2x + 3