Ответ:

а)

\[\frac{63a^6b^4}{c^7}\cdot\left(-\frac{c^3}{36a^5b^7}\right)=-\frac{63ac^3}{36b^3c^7}=-\frac{7a}{4b^3c^4}.\]

б)

\[\frac{54x^7}{y^{10}}:(18x^4y^6)=\frac{54x^7}{18x^4y^{16}}=\frac{3x^3}{y^{16}}.\]

в)

\[\frac{4(x+t)}{t}\cdot\frac{3t^2}{(x-t)(x+t)}=\frac{12t}{x-t}.\]

г)

\[\frac{6(x-5)}{x+8}\cdot\frac{2(x+8)}{(x-5)(x+5)}=\frac{12}{x+5}.\]

Ответ: а) \(-\frac{7a}{4b^3c^4}\); б) \(\frac{3x^3}{y^{16}}\); в) \(\frac{12t}{x-t}\); г) \(\frac{12}{x+5}\).