Maxima and minima (trapezoidal gutter)

I have a problem sir.

h=1/2√4a^2-(b-a)^2

A=1/4(b+a)√4a^2-(b-a)^2

dA/db=1/4[(b+a)-2(b-a)/2√4a^2-(b-a)^2 + √4a^2-(b-a)^2]=0.

Question:
How did it become +√4a^2-(b-a)^2]=0?
How did it become zero? Why is there zero?