im currently using an Inspiron 5547. When i bought it, it had windows 8.1 then i had to change my os to linux because of my assignments. Now i would love to get back my windows but i have already overwritten the os.

question 1: how do i know my cd key for the previous os

question 2: where can i get the iso?

question 3:I heard somewhere that the bios knows the cd-key. wondering if that is true