The first term (a) is - 18
You add 5 to get to the next term. Or you can solve it by taking any 2 consecutive terms and find their difference.
Formula
d = t4- t3
Givens
t4 = - 3
t3 = - 8
Solution 1
d = t4 - t3 Substitute
d = -3 - ( - 8) Remove the brackets
d = -3 + 8 Combine
d = 5 Difference
Remark
Find the general formula
tn = - a + (n - 1)d Substitute
So term 20 = Example
t20 = -18 + (20 - 1)*5 Combine the inside of the brackets. Remove the brackets
t20 = - 18 + 19*5 Combine 19 and 5
t20 = -18 + 95 "Subtract"
t20 = 77 Answer