Answer:
26 + x = 20 + 27
26 + x = 47, so x = 21
27 + 23 = 50, so the number in the center square is 24.
27 + 24 + 21 = 72. The sums of the numbers in each row, column, and diagonal is 72.
The remaining numbers are
25 (top left square), 28 (center right square), and 22 (bottom center square).