Подставим $$h=-\frac{5}{7}$$ в выражение $$-h(7-h)-(8-h)^2$$:
$$-(-\frac{5}{7})(7-(-\frac{5}{7}))-(8-(-\frac{5}{7}))^2 = \frac{5}{7}(7+\frac{5}{7})-(8+\frac{5}{7})^2 = \frac{5}{7}(\frac{49+5}{7})-(\frac{56+5}{7})^2 = \frac{5}{7} \cdot \frac{54}{7} - (\frac{61}{7})^2 = \frac{270}{49} - \frac{3721}{49} = \frac{-3451}{49}$$
Ответ: $$\frac{-3451}{49}$$