we know that
In the Arithmetic Sequence the formula is:
an= a1 + d(n−1)
where
a1 is the first term
d is the common difference
n is the number of terms
in this problem
a1=5
a2=18
a3=31
a4=44
a5=57
so
d=a2-a1 or a3-a2 or a4-a3 or a5-a4
d=57-44----> 13
find a6
a6= a1 + d(n−1)
n=6
d=13
a1=5
a6=5+13*(6-1)----> a6=5+13*5----> a6=70
the answer is
a6=70
