It's forming an A. P. because each succeeding Te rm increases by a common value i. e. 6 here.
The A. P is
22,28,34,.....
So the n th term will be represented as
a+(n-1)d,
where a=22
And d=6
Now in (b) part the n th term is given i. e. 100
So by putting in the given values we could easily found the value of n
n=(100-22)/6=78/6=13
So the 13th term will be 100
Hope You Got It.