Step 1: Complete the corners with smaller numbers 1, 2 and 3.
Step 2: The side having smallest numbers 1 & 2 are to be filled with the greatest number 6, the second smallest 1 & 3 side to be filled with the second largest 5 at the middle and so on.
The magic sum is 1 + 6 + 2 = 2 + 4 + 3 = 3 + 5 + 1 = 9.
Some other ways are given below.
The magic sum = 1 + 6 + 3 = 3 + 2 + 5 = 5 + 4 + 1 = 10.
The magic sum 6 + 1 + 4 = 4 + 5 + 2 = 2 + 3 + 6 = 11.
The magic sum 4 + 3 + 5 = 5 + 1 + 6 = 6 + 2 + 4 = 12.