Answer:
We get the value of Principal amount i.e initial investment = $61640
Step-by-step explanation:
Interest rate r = 3.59% or 0.0359
Compounded quarterly n = 4
Future Amount A = 117,300
Time t = 18 years
We need to find initial investment i,e Principal Amount P
The formula used is: 
Putting values and finding P

So, We get the value of Principal amount i.e initial investment = $61640