I believe that the answer you are looking for is (x-5)²-39, if you use the completing the square method.
Solution:
x²-10x=14
Transferring 14 from the right side to the left side,
x²-10-14=0
Squaring the equation,
(x-5)²-39, or (x-5)²=39 (Depends on how you prefer to write it).
So, that is the final answer. I hope this helped. If you have any questions about the answer, please let me know. Enjoy your day and take care!