Maturity value is given by

where: P is the initial value of the promissory note, r is the simple interest rate, and t is the time period (in years).
Given that
Webster Digital received a promissory note of $8,000 for 9 months at 7% simple interest from one of its customers.
P = &8,000; t = 9 months = 9 / 12 = 0.75 years; r = 7% = 0.07

The proceed from a discounted promisory note is given by

where: FV is the maturity value of the promissory note, d is the discount rate and t is the number of years remaining from the time the note was discounted to the maturity date.
Given that after 4 months, the note was discounted at Bank of Aventura at a discount rate of 10%.
t = 9 months - 4 months = 5 months = 5 / 12 years; d = 10% = 0.1; FV = $8,420

Therefore, the proceed is $8,069.17