(12-x)(10-x)x2=216 It is your first line. Write it in this way: (12-x)*(10-x)*x = 216.
Do not use "x" as the multiplication symbol.
Next step is to divide both sides by 2 to get (12-x)*(10-x) = 108.
It is equivalent to 120 - 22x + x^2 = 108.
Now you see that your second line (and all that follows) is WRONG.