answer 10
This is the formula of an arithmetic sequence.
a
n
=
a
1
+
d
(
n
−
1
)
a
n
=
a
1
+
d
(
n
-
1
)
Substitute in the values of 
a
1
=
1
a
1
=
1
 and 
d
=
3
d
=
3
.
a
n
=
1
+
(
3
)
(
n
−
1
)
a
n
=
1
+
(
3
)
(
n
-
1
)
Simplify each term.
a
n
=
1
+
3
n
−
3
a
n
=
1
+
3
n
-
3
Subtract 
3
3
 from 
1
1
.
a
n
=
3
n
−
2