Answer
n-th term:$\quad a_{n}=1.2+7.8n$,
The next term is $\quad a_{5}=40.2$
Work Step by Step
Note that
$16.8-9=7.8,$
$24.6-16.8=7.8,$
$32.4-24.6=7.8$
.... and we can also rewrite the first term as $1.2+7.8$
(to fit the pattern)
$n=1,\quad 9=1.2+7.8$
$n=2,\quad 16.8=9+7.8\quad=1.2+2\times 7.8$
$n=3,\quad 24.6=16.8+7.8=1.2+3\times 7.8$
$n=4,\quad 32.4=24.6+7.8=1.2+4\times 7.8\quad...$
So, the nth term is $a_{n}=1.2+n\times 7.8$,
and the next term is $a_{5}=32.4+7.8=40.2$
(also, $1.2+5\times 7.8=40.2$ )
