The formula for the future value of the account is
A = P(1 + r/n)^(nt)
where you have
A = 19909.20
P = 8975
r = 0.038
t = 21
The resulting equation is not one that can be solved by algebraic means, but we can use a graphing calculator to find n. This graph shows us n = 12, so
the interest compounds monthly.
