WINDOWS 10 did an update, which caused this neophyte a lot of hassle (and learning) trying to set the mutli-monitors back up the way they were; which I have accomplished.

Using NVIDIA GeForce 6100 with 4 monitors:

2, 1, 3 with #4 situated directly under #1. Monitor 2 is primary.

It used to be that we would stretch accross windows 1 through 3 to operate the sim. Now, we stretch accross monitors 1 thru 3; and the window stretches just fine, but FSX only fills Monitor #2 with No. 1 and 3 being black blank.

What do I need to do to get FSX to utilize the full stretched window, like it did before MS updates? 

[duplicate post]

Okay, let me start from the beginning: The guy who set this up for the museum (shared computer) doesn't want to maintain it anymore, so we are all in over our heads, here ....

1. FSX Acceleration (which it updated to after deleting fsx.cfg to solve crashing issue) on Win 10
2. 3 monitors: DISPLAY.Device.NVIDIA GeForce 6100.0] TriLinear=1 WideAspectView=True and running in Windows mode with all monitors set to "Aspect ratio" [all of this requiring some learning on my part, LoL]
3. Windows did an update, which resulted in having to set up all 3 monitors, which are now set is: 2, 1, 3 w/ 4 being directly under #1 and 2 set as primary [which took some more learning on my part, LoL]
4. (2) accounts; I am trying to make changes in a "user" account whether than an "admin" account (which might be a part of my problem!)
5. Read somewhere online about "extending desktop to this monitor"; the "extend" option is ghosted out and unselectable;
6. When booting up, only primary monitor shows logon and Windows Startup with the other monitors being black; the other monitors come to life after logging on (which is not the way it used to be; system used to use all monitors)
7. When launching FSX, I can stretch the window as wide as I want to but FSX will only utilize the window space that would fill up only one monitor.

Fixed!

Just for future reference, how?