\[\frac{26x^{7}}{51y^{5}} \cdot \frac{34y^{3}}{39x^{4}} = \frac{26x^{7} \cdot 34y^{3}}{51y^{5} \cdot 39x^{4}} =\]
\[= \frac{2x³ \cdot 2}{3y² \cdot 3} = \frac{4x^{3}}{9y^{2}}\]