3c(4c-5)-(c-8)(c-7)

= 3c(4c) - 3c(5) - c(c) + c(8) + c(7)

= 12c^2 - 15c - c^2 + 8c + 7c

= 11c^2 - 15c + 15c

= 11c^2

Поэтому 3c(4c-5)-(c-8)(c-7) = 11c^2.