Подставим $$n=-\frac{8}{13}$$ в выражение $$-\pi(7-n)-(n+3)^2$$:
$$-\pi(7-(-\frac{8}{13}))- (-\frac{8}{13}+3)^2 = -\pi(7+\frac{8}{13})-(\frac{-8+39}{13})^2 = -\pi(\frac{91+8}{13})-(\frac{31}{13})^2 = -\pi(\frac{99}{13})-(\frac{961}{169}) = -\frac{99}{13}\pi-\frac{961}{169}$$
Ответ: $$-\frac{99}{13}\pi-\frac{961}{169}$$