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
