Question 839
let x,y,z are no of shares in Delta Airlines, Hilton Hotels and Mcdonalds respectively.
2 days ago shares went doem by 350
therefore -1*x + (-1.5)*y + 0.5*z = -350
yesterday shares went up by 600
therefore 1.5*x + (-0.5)*y + 1*z = 600
you have 2 equation but 3 unknown, therefore cannot be solved.

since z=200
therfore 
-x-1.5y+0.5*200 = -350
-x-1.5y = -350 - 100 = -450
x+1.5y = 450  ----(1)
other one
1.5x-0.5y+200 = 600
1.5x-0.5y=400
let's multiply the equation by 3
4.5x-1.5y = 1200 -----(2)
(1)+(2) => 5.5x = 1800
              x = 327

sub x= 327 in (1)
327+1.5y=450
1.5y= 123
y=82