If the fourth term of an arithmetic sequence is 18 and twentieth term is 98, find the first term (a) and the common difference (d)

First Term = -12 ; Common difference = 10

Explanation:

a4 = a1 + 3d > 18 = a1 + 3d 
a12 = a1 + 11d> 98 = a1 + 11d 

8d = 80
d  =10       <<<<<<<<<<<<<  ¯\_(ツ)_/¯ We found the common difference

18 = a1 + 3(10) 
18 = a1 + 30
a1 = -12     <<<<<<<<<<<<<  ¯\_(ツ)_/¯ We found the first term