5) (7y-5)/11y-(10y-9)/11y+(10-5y)/11y=
(7y-5-10y+9+10-5y)/11y=(-8y+14)/11y
6) (21a+2b)/6a+(21a-3b)/6a-(36a-b)/6a=
=(21a+2b+21a-3b-36a+b)/6a=
=6a/6a=1
