#### Answer

$6(a-10)(a+2)$

#### Work Step by Step

Factor out $6$ to have:
$=6(a^2 -8a-20)$
The leading coefficient of the trinomial factor is 1.
To factor the trinomial, just look for the factors of the constant term (-20) whose sum is equal to the coefficient of the middle term (-8).
These factors are $-10$ and $2$.
Thus, the completely factored form of the given expression is:
$=6(a-10)(a+2)$