If $8700 is invested at 3% annual simple interest, how much should be invested at 6% annual simple interest so that the total yearly income for both investments is $393?

First off, see  how much 8700 as principal, yields at 3% APR
that is 

it will yield some amount

subtract that amount from 393
the difference is how much the yield will be on the 6% investment
so

It’s going to be d. 4x^2-4x-19’