15. \(\left(-\frac12x^3y^4\right)^2\cdot8xy^5\)

\(\left(-\frac12x^3y^4\right)^2=\frac14x^6y^8\).

\(\frac14x^6y^8\cdot8xy^5=2x^7y^{13}\).

Ответ: \(2x^7y^{13}\)