a) Решим уравнение $$2.1x + 3.5 = 1.4x$$:
$$2.1x - 1.4x = -3.5$$
$$0.7x = -3.5$$
$$x = -3.5 / 0.7$$
$$x = -5$$
б) Решим уравнение $$2(4 - 1.9x) = 0.8 - 0.2x$$:
$$8 - 3.8x = 0.8 - 0.2x$$
$$-3.8x + 0.2x = 0.8 - 8$$
$$-3.6x = -7.2$$
$$x = -7.2 / -3.6$$
$$x = 2$$
Ответ: a) $$x = -5$$, б) $$x = 2$$