What do you want it to look like? (A large decimal number, a number with commas, a number with periods as separators, a number with fewer decimal places, feet and inches...........
5.975308620246914E+016. to a simpler number.
It would be simpler if you gave the entire description.
If it is a teest string, assigned to a variable,
in this example, 'rns, then to function 'rtos would:
(setq rns "5.975308620246914E+016")
(setq RN (read rns))
(Princ "\n Read String: ") (prin1 RN)
(setq pns (rtos rn 2 2))
(Princ "\n Real-To-String: ") (prin1 pns )