Staircase Sum

The digits 1-9 have been written in the squares so that each row and column sums to 13. What is the value of n?
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Problem

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Staircase Sum
 

The digits 1, 2, 3, 4, 5, 6, 7, 8 and 9 are to be written in the squares so that every row and every column of three squares has a total of 13. Two numbers have already been entered. What is the value of n?

If you liked this problem, here is an NRICH task that challenges you to use similar mathematical ideas.