Remark
The difference is between any two terms. You should pick t3 and t4 which are both positive. That's easier to handle than a couple of minus signs.
d
t4 - t3 = d
13 - 5 = d
8 = d
a
the first term is - 11
Formula
t_n = a + (n-1)*d
t_n = -11 + (n-1)*8
Check
t_n = - 11 + (3-1)*8
t_3 = - 11 + 16
t_3 = 5 which it does.
Try t_5. It should be 21
t_5 = - 11 + (5 - 1)*8
t_5 = - 11 + 4 * 8
t_5 = -11 + 32
t_5 = 21
Answer
t_n = -11 + (n - 1)*8