5x-ky=0 => 5x=ky
1：若 x = y ,则 k = 5
则 3x + 5y = 8x = 4
得 x = y = 0.5,k = 5
2：若 x = k,则 y = 5
则 3x + 25 = 4
得 x = k = -7,y = 5