The 4th term from the end of an AP - 11, - 8, - 5, …., 49 is
First term, a = - 11
Common difference, a2 -a = - 8 - (- 11) = 3
Let the last term be an
an = a + (n - 1)d
49 = - 11 + (n - 1)3
49 + 11 = (n - 1)3
n - 1 = 60/3
n - 1 = 20
n = 21
4th term from last will be 18th term from starting
And a18 = a + 17d
= - 11 + 17(3)
= 40
Alternatively we can use the direct formula
an = l - (n - 1)d
where, an = nth term from last of an AP
l = last term
d = common difference