6. 6,8z² + (5,2 – z²) – (4,7z² – 7)

6. $$6,8z^2 + (5,2 - z^2) - (4,7z^2 - 7) = 6,8z^2 + 5,2 - z^2 - 4,7z^2 + 7 = 1,1z^2 + 12,2$$

Ответ: $$1,1z^2 + 12,2$$