an arithmetic sequence, means that there is an equal difference between each number of the sequence.
i.e. a, a+d, a+2d, a+3d...
where a is the first term, and d is the difference
in your case, a (first term) = 58
to find the difference, subtract 58 (a) from 69 (a+d) to get d.
d = 69 - 58 = 11
(you can always check your calculation on another two consecutive terms: 91 - 80 also equals 11)
the general formula for any term in an arithmetic sequence is:
T(n) = a + (n-1)d ...(or A(n))
in your case, n = 21
subbing into the formula:
A21 = a + (21-1)d = 58 + 20(11) = 278
hope that helps :)
