Answer
$C(t)= 30t+90$,
$7$ months
Work Step by Step
Let $t$ = months of membeship.
Then $C(t)= 30t+90$
and the t for which $C(t)=300$ is found from
$300= 30t+90\qquad$ ... subtract 90
$ 210=30t\qquad$ ... divide with 30
$t=7$ months