Lucia finds that (3x^2+3x+5) + (7x^2-9x+8) = 10x^2-12x+13. what error did lucia make?

The answer 
to do the calculus, the method is 
taking off the paranthesis
adding the term with a similar degree
summing constant terms

 (3x^2+3x+5) + (7x^2-9x+8) = 

  = 3x^2+3x+5 +  7x^2-9x+8
  
  = 3x^2 +  7x^2  + 3x+5 - 9x+8
 = 10x²  + 3x - 9x +5 +8 
 = 10x² - 6x + 13
the answer is 
(3x^2+3x+5) + (7x^2-9x+8) =10x² - 6x + 13

the error that Lucia did make  3x - 9x = -12x, instead of 3x - 9x = -6x