$800 is deposited in an account that pays 9% compounded semi-annually. find the balance after 4 years.

The formula for compound interest is the following:

A=P(1+r/n)^nt
A=accumulated amount (what we're looking for)
P=Principal amount (initial amount). $800 in this case
r=rate. 0.09 in this case which we get from converting 9% to decimal by dividing by a 100.
n=number of times interest is compounded. In this case semi-annually which means 2
t=time. In this case 4 years
Let's calculate:
A=800(1+0.09/2)^(2*4)
A=800(1+0.045)^8
A=800(1.045)^8
A=800(1.42210061284)
A=1137.68049027
Let's round to the hundredth place (to represent cents) since the amount represents money.
Answer=The balance after 4 years will be $1,137.68