For each of the following, simplify by combining like terms :
a) p ( q - r ) - q ( p -r ) - r ( p -q )
b) 23ab - 12bc^2 - 12ba - 13bc^2
pq-pr-qp+qr-rp+rq
or, arranging factors alphabetically,
pq-pr-pq+qr-pr+qr
-2pr+2qr
23ab-12ab - 12bc^2-13bc^2
11ab - 25bc^2