#### Answer

$-15$

#### Work Step by Step

RECALL:
$a-b=a+(-b)$
Thus,
$-6-9=-6+(-9)$
The numbers being added have the same sign.
Perform the following steps:
(1) Add their absolute values.
$|-6| + |-9| = 6+9 = 15$
(2) The sum will have the same sign as the original numbers.
Thus,
$(-6)+(-9) = -15$