\[\frac{3x}{4x - 4} + \frac{5x}{7 - 7x} =\]
\[= \frac{3x^{\backslash 7}}{4(x - 1)} - \frac{5x^{\backslash 4}}{7(x - 1)} =\]
\[= \frac{21x - 20x}{28(x - 1)} = \frac{x}{28x - 28}.\]