Let us formulate a set of equations, being X, Y, V, Z values to fill in the spaces
We can write four equations as follows:
X + Y = 8 ( 1)
V - Z = 6 (2)
X + V = 13 (3)
Y + Z = 8 (4)
By adding equations (2) and (4) V + Y = 14
By multiplying (1) per -1 and adding to equation (3) V - Y = 5
By adding the two later equations 2V = 19 ; V = 9,5
V - Z = 6 ; Z = 3,5
X + 9,5 = 13 ; X = 3,5
3,5 + Y = 8 ; Y = 4,5
Then, the solution is
( 3,5 ) (4,5)
(9,5) ( 3,5)