Заданное уравнение линейное:

3.5x - 2.3x + 3.8 = 4.28;

3.5x - 2.3x = 4.28 - 3.8;

x(3.5 - 2.3) = 0.48;

1.2x = 0.48;

x = 0.48 : 1.2;

x = 0.4.