If 7800 is invested at 5.2 interest compounded monthly, how much will the investment be worth in 11 years

To add 3.5% to a number you simply multiply the number by 1.052. To compound that amount year after year, you raise 1.052 to an exponent that corresponds to the number of years. 7800 * (1.052^11) = 13622.8426605