first add
(3a²-5a+5)+(a³-5a²+8)
=3a²-5a+5+a³-5a²+8
=a³-2a3a²-5a²+5+8
=a³-2a²+13
and now,
substract a³-2a²+13 from a²-4a²-9
=7a²-4a²-9-(a³+13)
=7a²-4a²-9-a³+2a²-13
= a³+7a²-4a²+2a²-9-13
=-a³+5a²-22
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