the error is in line#1:
9-24 = -15
25-40 = 15
While techically (meaning the way the equation is being treated with completing the square, radicating [correct word??] etc.) the procedure is correct, you start the "proof" under the false premise that -15 = +15
This triggers a cascade of errors with the rest of the calculation, much like a domino-effect:
In Line #2 you get 1 = 31
In Lines #3 and #4 the equation reads as -1 = +1
The important thing - both in mathematics and in life - is to never take anything for granted but always to check the supposed facts yourself, without blindly relying on others.