Slide: $12$

Ben is required to pay back a loan in two instalments: $$2800$ in $5$ months and $$4000$ in $8$ months. He realized that he would not have the money to repay these amounts as scheduled but will be able to clear the loan with one single payment in $15$ months. What would be the value of this single payment if interest on the loan is $6\mathrm{.}75\%$ p$.$a$.?$ Use $15$ months as the focal date.