1) (6х²-5х+9)-(3х²+х-7)=6х²-5х+9-3х²-х+7=6х²-3х²-5х-х+9+7=3х²-6х+16;
2) (5а²-2а-3)-(2а²+2а-5)=5а²-2а-3-2а²-2а+5=5а²-2а²-2а-2а-3+5=3а²-4а+2;
3) (9у²-5у+7)-(3у²+2у-1)=9у²-5у+7-3у²-2у+1=9у²-3у²-5у-2у+7+1=6у²-7у+8;
4) (7в²-4в+2)-(5в²-3в+7)=7в²-4в+3-5в²+3в-7=7в²-5в²-4в+3в+2-7=2в²-в-5;