$9=11$ !!!

Level 2

Line 1: $99=99$

Line 2: $180-81=220-121$

Line 3: $81-180=121-220$

Line 4: $9^2-2\cdot9\cdot10+10^2=11^2-2\cdot11\cdot10+10^2$

Line 5: $(9-10)^2=(11-10)^2$

Line 6: $9-10=11-10$

Cancelling $-10$ from both sides, we obtain $9=11$

In which line I did the mistake?

9=119=11 9=11!!!