6.18 was incorrect . please explain your answer.

Find the annual effective interest rate which is equivalent to the force of interest of 8 = 0.09 -0.03t during the time interval between t-1 and t=3. The unit of t is a year. Round the answer to the nearest hundredth percent. (E.g., input 1.23 if the answer is 1.234%.) 6.18