Hello, thank you for taking the time to post your question!
On a question like this you want to take the given value and then divide by the percentage in order to get the total.
That would mean taking
3990 women / 95% = 3990 / 0.95 = 4200 total undergraduates
I hope that helps get you moving in the right direction! Feel free to reach out if you have any additional questions beyond that :)