Determine the amount of interest earned on $6400 invested at 4.8%, compounded semi-monthly, for 7 years.

We know that 
 In the Arithmetic Sequence, the  formula is
an=a1+(n-1)*d
where

an = the term that you want to find

a1 = first term in the list of ordered numbers

n = the term position
d = common difference of any pair of consecutive numbers

in this problem we have
 a1=34
d=15
so
for n=200
a200=34+(200-1)*15> a200=3019

the answer is
3019