How do i find the x and y?

Dakota earned $7.50 in interest in account a and $18.75 in interest in account b after 15 months. if the simple interest rate is 4% for account a and 5% for account b, which account has the greater principal? explain. to make it a little easier, there is an image. good luck!

Rewrite so the subject can be y in the formula d=m-y/y+n

What adds to be the bottom number but also multiplies to be the top number with 8 on the bottom and 15 on top