Most probably it used to have Win 7 Home, 64-bit and first boot could be set to the DVD drive. Searching Google there are various models of the L750D. See click here , click here , click here etc. Haven't gone through them all but it certainly sounds as if it has been upgraded to windows 10 Home 64-bit.
If you wan't to boot from USB search Google how to get to Bios/Uefi to set USB as first boot. You could also download Win 10 Home 64-bit from here and burn the .iso file to a DVD disk and do a fresh install. A key won't be needed as it is already registered with Microsoft. Assuming original is not a pirated version. You'll have to do a bit of homework though as I can only guess and suggest ideas from your information. Good luck.