А)

3 + x != 0 => x != -3

x - 3 != 0 => x != 3

Б)

x != 0

x^2 - 4x + 3 != 0 => (x - 1)(x - 3) != 0 => x != 1 ; x != 3

В)

x - 3 != 0 => x != 3

x^2 - 3x != 0 => x(x - 3) != 0 => x != 0 ; x != 3

Г)

x - 3 != 0 => x != 3

3 - x != 0 => x != 3

Д)

2x - 3 != 0 => 2x != 3 => x != 3/2 => x != 1.5

Е)

x + 3 != 0 => x != -3

x^2 - 3x != 0 => x(x - 3) => x != 0 ; x != 3

x^3 - 9x != 0 => x(x^2 - 9) != 0 => x != 0 ; x^2 != 9 => x != 0 ; x != 3 ; x != -3

P.S. за лучший ответ + 10 к карме :)