Greg has used triangle property for this question.
My approach:
The middle line is y=x –> 45 degree
(a,b) is below y=x & (c,d) is above y=x.
According to the reflection rule, (x,y) = (y,x) for the line y=x.
So can we tell a+b = c+d?
Yes this approach works for this question.
But keep in mind that in this case the points were symmetrical about the y=x line.
If the angles were different, that might not be the case which is when you apply the triangle property.