windows 7 product key problem

hi all,

i have a probook 4520s, it came with a windows 7 pro. however, i formated the labtop and installed the winodws 8 pro to try it, and now i want to get back to windows 7. But the problem is that the label that contains the product key at the bottom of the labtop is not clear and has been erased.

so how can i get back the product key?

thank you all.