Answer
$a_{n} = (-1)^{n} 9n$
Work Step by Step
Given sequence is $-9,18,-27,36,...$
$a_{1} = -9 = (-1)^{1} \times 9 \times 1 $
$a_{2} = 18 = (-1)^{2} \times 9 \times 2 $
$a_{3} = -27 = (-1)^{3} \times 9 \times 3 $
$a_{4} = 36 = (-1)^{4} \times 9 \times 4$
Similarly,
$a_{n} = (-1)^{n} \times 9 \times n $
$a_{n} = (-1)^{n} 9n$
