Any triangle number plus the previous is the square of the longest side of the first triangle number

"The first three letters in the grid are: e,n,d

Because everything is "en"ed."

This may refer to the fact that everything is placed in the grid with e as the x axis and n as the y axis. It may also refer to something else, but this seems like the most likely explanation.

There have been a few suggestions as to the order in which we need to focus on particular things, one of which is the following:

"Breaking the problem down.

First by roots.

Then by triangular numbers."

The roots in this context would appear to be (but not necessarily be) the tree made from d and e.