Answer
$a_{n}=(n+1)^{2}-1$
$a_{5}=35$
Graph:
.
Work Step by Step
We may recognize a pattern such as
$n=1,\quad a_{1}=3=4-1=2^{2}-1$
$n=2,\quad a_{2}=8=9-1=3^{2}-1$
$n=3,\quad a_{3}=15=16-1=4^{2}-1$
$n=4,\quad a_{4}=24=25-1=5^{2}-1$
The pattern that emerges is that $a_{n}=(n+1)^{2}-1$
The next term would be $a_{5}=6^{2}-1=35$
To graph, plot the points (1,3), (2,8), (3,15), (4,25) (5,35).