2021 Cayley Problem 22

The cells of a 3 \times 3 grid are to be filled with integers so that the average value of the entries along each row, each column, and each diagonal is the same. The integers 10, 64 and 70 are entered, as shown. When the remaining six squares are filled in to complete the grid, what integer replaces x?

\begin{array}{|c|c|c|} \hline 64 & 10 & \\ \hline 70 & & \\ \hline & x & \\ \hline \end{array}

\textbf{(A)}\ 78\quad \textbf{(B)}\ 82\quad \textbf{(C)}\ 86\quad \textbf{(D)}\ 90\quad \textbf{(E)}\ 94

If there are no answer choices shown, enter a numerical answer.

Full credit to this problem is given to the CEMC, you may view all Cayley contests here.