jmc

Denote the remaining entries by $d,e,f,g,h,$ as shown: One possible solution proceeds in three steps, as follows:

The leftmost column and up-right diagonal have the same sum, so $x+1+f=96+d+f,$ which gives $d=x-95.$ The down-right diagonal and the rightmost column have the same sum, so $x+(x-95)+h=96+e+h,$ which gives $e=2x-191.$ Finally, the first row and second sum have the same sum, so $$x+19+96=1+(x-95)+(2x-191),$$which gives $x=\boxed{200}.$