[tex]\displaystyle\bf\\\frac{84ac^{3} -315bc^{5} }{490bc^{5} } =\frac{84a\cdot c^{3} -315bc^{2} \cdot c^{3} }{490bc^{2} \cdot c^{3} } =\\\\\\=\frac{c^{3} \cdot(84a-315bc^{2} )}{490bc^{2} \cdot c^{3} }=\frac{84a-315bc^{2} }{490bc^{2} }[/tex]