How many terms are in the arithmetic sequence 1313, 1616, 1919, …… , 7070, 7373?

Since we know this is an arithmetic sequence, to find the number of terms, it will be helpful to find the distance d between every term.
d = 1616 - 1313
d = 303
now let's think about how to generate the nth term of an arithmetic sequence given the first term 1313 and distance d. 
second term = d + 1313
third term = second term + d
third term = 1313 + 2d
fourth term = third term + d
fourth term = 1313 + 3d
we are starting to see a pattern:
nth term = 1313 + (n-1)d 
we know this pattern will continue because we are simply adding d to the previous term to get the next term so if the previous term n - 1 was 1313 + d(n - 2), then the next term n will be 1313 + d(n-1).  to put i simply, since the first term was 1313, we know the 2nd term must be 1313 + d and since the 2nd term is 1313 + d, we know the third term must be 1313 + 2d and so on until the nth term which is 1313 + (n-1)d.  we know the nth term of this sequence is 7373 and we know that d is 303 so:
7373 = 1313 + 303(n-1) and we just have to solve for n.
doing so gives:
n = 21 terms

let me know if you have any questions!