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)