(1)(3a²-5a+6)-3(-2a²-2a+4)
=3a²-5a+6+6a²+6a-12
=9a²+a-6
(2)3a²b-[ab²-2(a²b-2ab²)]
=3a²b-[ab²-2a²b+4ab²]
=3a²b-[5ab²-2a²b]
=3a²b-5ab²+2a²b
=5a²b-5ab²
(3)3a²-[7b-(4a-b)-2a²]
=3a²-[7b-4a+b-2a²]
=3a²-[8b-4a-2a²]
=3a²-8b+4a+2a²
=5a²+4a-8b
(4)-7x²+(6x²-5xy)-2(3y²+xy-x²)
=-7x²+6x²-5xy-6y²-2xy+2x²
=x²-7xy-6y²