the error is in line#1:
9-24 = -15
25-40 = 15
You start the "proof" under the false premise that -15 = +15 which triggers a cascade of errors with the rest of the calculation, much like a domino-effect.
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.
Actually, the first line is correct.
Given the leap of adding 16 to each side in statement 2, the transition from level 2 to level 3 is also correct.
I think the main mistake is in the 4th statement - both sides should be divided by the same common multiplier to keep things equal, but instead the left is divided by (3-4) and the right by (5-4).
Taking the rest of the sequence beyond that error, you still end up with a difference of 2 instead of equality, whether you physically subtract 4 from each side or just let them cancel each other out.
Do correct me if I am wrong - I've just woken up.