# A simple optimization problem [duplicate]

I am trying to derive ELM going through the basics , please help me out here :

$$f = x^Tx$$ $$g = Ax-b$$

The constraint is $Ax-b = 0$

I calculated $J' = f'+\lambda^T g'$ which is $2x+(\lambda^T A)^T = 0$ and $Ax-b=0$ . I dont know what to do next please help me out .