Nexus 6, stock image, fastboot not a command

Hey guys I have recently bought a nexus 6 and my mate tried to root it with kingroot, as he did so without me knowing, I could not update my nexus 6 at all, I would get an error every time, ive got my image from Google developers but when I run flash all.bat I am greeted with "fastboot is not an internal or external command" several times... Can someone help






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