First we should find out
"how much does a^3 - 2a exceed a^2 + a - 6 ?"
this would be
(a^3 - 2a) - (a^2 + a - 6)
we distribute the negative
a^3 - 2a - a^2 - a + 6
a^3 - a^2 - 3a + 6
(I made it already decending in powers out of habit, note that it is a^3 followed by a^2 then a etc)
therefore a^3 - a^2 - 3a + 6 would be your answer
