原式=2a-{5b+[2a-3b-(-6a-5b)]}
=2a-[5b+(2a-3b+6a+5b)]
=2a-[5b+(8a+2b)]
=2a-(5b+8a+2b)
=2a-(8a+7b)
=2a-8a-7b
=-6a-7b