find the time required for an investment of 5000 dollars to grow 9000 dollars at an interest rate of 7.5 percent per year compounded quarterly.

It is compounded at a rate of per quarter, so after each quarter you will have of your original investment.

so your equation should be.

As the investment is compounded quarterly and not continuously it would not make sense to submit an answer like 1.345 (I am not suggesting this is the answer, as it is not). Only answer of the form x.25 x.5 or x.75 make sense.

Hope this helps

Bobak