The numbers $2$, $3$, $4$, $5$, $6$, $7$ and $8$ are placed in the squares in this diagram.
The four numbers in the horizontal row add up to $21$, and the four numbers in the vertical column also add up to $21$.
Which number should replace $x$?
If you liked this problem, here is an NRICH task
that challenges you to use similar mathematical ideas.