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Let the missing numbers be x, y, a and b

x + y = 8 ---- (1)
a - b = 6 ---- (2)
x + a = 13 --- (3)
y + b = 8 ---- (4)

2x + a + y = 21 --- From (1 & 3)
a + y = 14 ----  From (2 & 4)

Substitute (2 & 4) in (1 & 3)

2x + 14 = 21
2x = 7
x = 3.5

From (1) ---> 3.5 + y = 8
=> y = 8 - 3.5 = 4.5

a + y = 14 => a = 14 - y = 14 - 4.5 => a = 9.5
From (4) ---- y + b = 8 => b = 8 - y = 8 - 4.5 = 3.5

Hence,
x = 3.5
y = 4.5
a = 9.5
b = 3.5