The answer can be obtained by subtracting from $$ 2a^{3} + 5a - a^{2} - 6 $$ to get $$ a^{2} - a - a^{3} + 1 $$
$$ = \left ( -a^{3} + a +1 \right ) - \left ( 2a^{3} + 5a - a^{2} - 6 \right ) $$
It can be written as
$$ -a^{3} + a^{2} - a + 1- 2a^{3} - 5a +a^{2} + 6 $$
By further calculation
$$-a^{3} + 2a^{2} - 6a + 7 $$