m{x-(1+√2)}(x-a)
= m{x^2 -(1+√2+a)x +(1+√2)am}
= mx^2 -(a+√2+a)mx +(1+√2)am

2x^2 -4x +k と係数比較
m=2 , -4=-(1+√2+a)m , (1+√2)am=k

4=(1+√2+a)2
2=1+√2+a
a=1-√2

(1+√2)am =k
(1+√2)(1-√2)2 =k
(1-2)2 =k
-2=k