$$9a^3+4a^2 + 5a-5 .$$
Work Step by Step
We simplify by removing the parentheses and then add the like terms as follows: $$ (2a^3 - a) + (4a^2 + 7a) + (7a^3 - a -5)\\ =2a^3 - a + 4a^2 + 7a+ 7a^3 - a -5\\ =9a^3+4a^2 + 5a-5 .$$
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