5.122 Найдите корень уравнения и выполните проверку: a) -30(x – 21) = -180; б) (15 – 9x)4 = 204; в) $$\frac{9}{4}x - \frac{5}{14} = \frac{1}{7}$$.

a) -30(x – 21) = -180 x - 21 = -180 / -30 x - 21 = 6 x = 6 + 21 x = 27 б) (15 – 9x)4 = 204 15 - 9x = 204 / 4 15 - 9x = 51 -9x = 51 - 15 -9x = 36 x = 36 / -9 x = -4 в) $$\frac{9}{4}x - \frac{5}{14} = \frac{1}{7}$$ $$\frac{9}{4}x = \frac{1}{7} + \frac{5}{14}$$ $$\frac{9}{4}x = \frac{2}{14} + \frac{5}{14}$$ $$\frac{9}{4}x = \frac{7}{14}$$ $$\frac{9}{4}x = \frac{1}{2}$$ x = $$\frac{1}{2} \cdot \frac{4}{9}$$ x = $$\frac{2}{9}$$