Hints:

1. Biggest Sum – 28:

   • The circle with the sum 28 is the highest.

   • It contains the numbers 5 + 6 + 8 + 9 = 28.

   • Importantly, this sum does not include 7, which helps eliminate possibilities.

2. Upper Right 2×2 Square:

   • In this 2×2 section, the largest values visible are 3 and 4.

   • This suggests that 8 or 9 must occupy the center square between the 7 and 5.

3. Try Two Scenarios:

   • Try solving with 8 in the central square.

   • Try solving with 9 in the central square.

   • Only one of them should work based on the other required values in nearby circles.