First of all you take LCM then you get
X^2 -2(a + b)X +3ab = 0
Then apply quadratic formula
X = -b +- ✓b^2 - 4ac/2a
Then you got 2 answers
1: X1 = a + b + ✓a^2 -ab + b^2
2: X2 = a + b - ✓a^2 -ab + b^2
According to the condition
X1 = -X2
After calculation you got
a+b = 0
Hope you understand