How do I solve?

Question

What expression must the center cell of the table below contain so that the sums of each row, each column, and each diagonal are equivalent?

 

  x        8x       -3x

 

-2x                  6x

 

 7x      -4x        3x

 

Answer 1

2x seems to do the trick.  Just a quick guess, though; I leave it to you to verify in all cases.