The first step I have done was to sum all the numbers of the diagonal line that stars in bottom left and ends in top right.
The result was: 23+6+11=40
So to be there a relation I tried the diagonal that started in the bottom right and ended in top left.
The result was= 20+6+?=26+?
Next I had subtracted 40-26=14 that is equivalent to J.
Next I have done the relation with the vertical line and added 14+23=37 and 11+20=31 there is no relation so I have tried to subtract and the results were 23-14=9 and 20-11=9
There is a relationship in the horizontal line.
Then with subtraction in the horizontal it worked 23-20=3 and 14-11=3 but there was no relation with addition 23+20=43 and 14+11=25.
The last relation I found was that the sum of the the bottom left and top right was with the same result as the sum of the bottom right and top left.