[tex] \frac{6a}{7c} - \frac{36 {a}^{2} + 49 {c}^{2} }{42ac} + \frac{7c - 36a}{6a} = \frac{36 {a}^{2} - 36 {a}^{2} - 49 {c}^{2} + 49 {c}^{2} - 252ac}{42ac} = \frac{ - 252ac}{42ac} = - \frac{252}{42} = - 6[/tex]

Ответ: 6