把x寫作equation y=ax^2+bx+c 的subject

Completing the square, also called perfect square.
Shown as follow:

y=ax^2+bx+c
y=a(x^2 + b/a x + c/a)
y=a[(x+b/2a)^2 - (b/2a)^2 +c/a]

Thus,
x=square root of [(y-c)/a+(b/2a)^2] - b/2a

y=ax^2+bx+c [ x ]
y-c=x(ax+b)
x = (y-c)/(ax+b)

as follows:
http://i299.photobucket.com/albums/mm309/lokwanshan/ScreenHunter_05Jun060008.gif