When did I conceive if I'm due April 9t

There is no exact date that can be obtained, At the very best you will get a give or take two weeks.

But it is normally at some point during that 4 week period after your last period.

So when was your last period ?

The end of May, first of April.
If you are wanting an exact date, it does not exist

When did I conceive if I'm due April 9t

Most likely somewhere between July 15-20. But you must remember that dating pregnancy is simply nothing more than an educated guess at best. If you are concerned about paternity, you will have to have a DNA test done once the baby is born.