M坐标是(-2,2)吧.
焦点(2,0),设直线：y=k(x-2)
令A(x1,y1),B(x2,y2),则：y1=kx1-2k,y2=kx2-2k
MA=(x1+2,y1-2),MB=(x2+2,y2-2)
MA·MB=0,即：(x1+2,y1-2)·(x2+2,y2-2)
=(x1+2)(x2+2)+(kx1-2k-2)(kx2-2k-2)
=x1x2+2(x1+x2)+4+k^2x1x2-2k(k+1)(x1+x2)+4(k+1)^2
=(k^2+1)x1x2+(2-2k^2-2k)(x1+x2)+4(k+1)^2+4=0
又：k^2(x-2)^2=8x,即：k^2x^2-4(k^2+2)x+4k^2=0
故：x1+x2=4(k^2+2)/k^2,x1x2=4
故：4(k^2+1)+(2-2k^2-2k)(4k^2+8)/k^2+4(k+1)^2+4=0
即：4k^4+4k^2+8k^2-8k^4-8k^3+16-16k^2-16k+4k^4+8k^3+8k^2=0
即：4k^2-16k+16=0
即：k^2-4k+4=0
即：k=2