To find a5, we can use the formula for an arithmetic sequence:

an = a1 + (n-1)d

where:
an is the nth term
a1 is the first term
d is the common difference between terms
n is the term number

In this case, a1 = 13, a2 = 9, and we need to find a5. We can find the common difference (d) first:

d = a2 - a1
d = 9 - 13
d = -4

Now we can find a5:

a5 = a1 + (5-1)d
a5 = 13 + 4(-4)
a5 = 13 - 16
a5 = -3

Therefore, a5 = -3.