Solution below.

Reveal solution by highlighting:

In the first row, we add 1-17. 1+17 = 18, 2+16=18, and so on. So this sum can also be written as 18+18+…+18+9. Because the 9 in the middle can’t be paired up. This can also be written as 8.5*2*9. (8 sums being 18 = 2*9 and the 9.)

The next row turns out to be 7.5*2*9, the next 6.5*2*9, all the way to 0.5*2*9. When we add all the rows, it can be written as (8.5+7.5+…+0.5)*2*9. Again, we can pair numbers: 8.5+0.5 = 9, 7.5+1.5 = 9, all the way to the middle, with 4.5 not paired up. So the parentheses can also be written as 4.5*2*4.5. The whole sum will be 4.5*2*4.5*2*9. This is also 9³=729.