Hi, I have attempted to solve this problem but cannot get the right answer. I got t= 7080fs and it was wrong and I'm not sure why.

Hi, I have attempted to solve this problem but cannot get the right answer. I got t= 7080fs and it was wrong and I'm not sure why. I found the velocity of each ray and then found the time to pass through the glass with t = L/Cglass and then found delta t to get my overall difference in time but it wasn't correct. Can you please help me?

Thanks!