Question

x+Y=9
x- y= 5
4x-y=8
x +y = 7

Answer 1

x+y=9
x=9-y

x- y= 5
(9-y)-y=5
9-y-y=5
9-2y=5
-2y=5-9
-2y=-4
-y=-4/2
-y=-2 *(-1)
y=2

O valor de Y será 2 
x=9-y --> x=9-(2) --> x=7

Basta substituir X e Y para tirar a prova real nas restantes!