b(b^2-x^2)=ax(a-x)-ab^2
b^3-bx^2=a^2x-ax^2-ab^2
(a-b)x^2-a^2x+b^2(a+b)=0
[(a-b)x-b^2][x-(a+b)]=0
x=b^2/(a-b),x=a+b