Answer: 200

Small Hint:

Let $e$ be the center entry and $S$ the common sum

Big Hint:

In a $3\times3$ magic square, $S=3e$ and opposite entries sum to $2e$

Solution:

Let the center entry be $e$ and the common sum be $S.$ In any $3\times3$ magic square, $S=3e$ and entries opposite across the center sum to $2e.$ Thus the bottom-left entry is $2e-96.$ The first column and first row give $$\begin{aligned}x+1+(2e-96)&=3e,\\x+19+96&=3e.\end{aligned}$$ The first equation says $e=x-95,$ while the second says $3e=x+115.$ Hence $3x-285=x+115,$ so $x=200.$