why is there an addition of v^3 to the second part of the equation of value

EXAMPLE 3.1 (Amortization table) A loan of amount 1000 at a nominal annual interest rate of 12% com- pounded monthly is repaid by 6 monthly payments, starting one month after the loan is made. The first three payments are amount X each and the final three payments are amount 2X each. Construct the amortization schedule for this loan. SOLUTION To solve for X we have 1000 = Xa3.01 +2Xvazon as the equation of value