Great kata, although I believe it is tough for 7kyu level.

You are probably trying a brute-force approach, looping over and getting the sum in each loop. Although your solution works with a small amount of data (i.e. smaller triangles, lower n number), it is not optimal for a large amount of data (i.e. bigger triangles, higher n number). You need find an algorithmic solution or an equation which will solve the problem easier than the brute-force.

This Kata is clearly not for 7kyu level, period. It contains knowledge beyond 7kyu as diggi55 stated.