Решите уравнения: a) $$\frac{3}{7}x = -33$$ b) $$0,4x + 2,1 = -0,3$$ в) $$3x - 4 = 5x + 21$$ г) $$6x - (2x + 5) = 4(8x - 3)$$.

a) $$\frac{3}{7}x = -33$$ $$x = -33 \cdot \frac{7}{3}$$ $$x = -11 \cdot 7$$ $$\boxed{x = -77}$$ b) $$0,4x + 2,1 = -0,3$$ $$0,4x = -0,3 - 2,1$$ $$0,4x = -2,4$$ $$x = \frac{-2,4}{0,4}$$ $$\boxed{x = -6}$$ в) $$3x - 4 = 5x + 21$$ $$3x - 5x = 21 + 4$$ $$-2x = 25$$ $$x = \frac{25}{-2}$$ $$\boxed{x = -12,5}$$ г) $$6x - (2x + 5) = 4(8x - 3)$$ $$6x - 2x - 5 = 32x - 12$$ $$4x - 5 = 32x - 12$$ $$4x - 32x = -12 + 5$$ $$-28x = -7$$ $$x = \frac{-7}{-28}$$ $$x = \frac{1}{4}$$ $$\boxed{x = 0,25}$$