What is the 21st term in the sequence:?

I use this equation as a general way of explaining the n'th term of an arithmetic sequence:

a(n) = a + b(n - 1)

where "a" is the first term (6)
and "b" is the common difference (4)

You want the 21st term (n), so we can substitute and solve for a(21):

a(n) = 6 + 4(n - 1)
a(21) = 6 + 4(21 - 1)
a(21) = 6 + 4(20)
a(21) = 6 + 80
a(21) = 86

Sequence 6, 10, 14, 18,...is an AP of form a, a+d, a+2d,....,a+(n-1)d where am = 1st
term = t(1), d = common difference between consecutive terms;
ie., d = {t(n+1)-t(n), n = 1,2,3,......} and t(n) = a+(n-1)d.;

Here, a = 6 & d = 4;
Then t(n) = 6+(n-1)4;
For n = 21, we have t(21) = 6+(20)(4) = 86;
Choice D gives correct answer.

the difference between two sequential terms is 4 so we can generalize this sequence as

S_n = 6 +4*(n-1)  for n = 1, 2, 3, 4, ...

S_21 = 6+4*20 = 86