I just installed Ubuntu 18.04 and went through loads of settings. Among them, I followed the steps in <https://help.ubuntu.com/stable/ubuntu-help/display-dual-monitors.html>

The screens do what I want... until I reboot. Then the displays settings are back at their defaults. How/where can I save my settings so that a restart with the same monitors will reload the settings for them?

Notes:
1) This was not a problem in 16.10 with Gnome desktop on same machine with same monitors. It is new (for me) only since installing 18.04

2) Machine is Acer laptop with Intel i5 CPU and its on-board HD620 graphics. The Intel graphics driver is installed, but I haven't yet figured out how to see if it's the driver in use. Monitors are built-in + attached flat-screen (attached throughout power-up, boot and login).

PS: If my question is a duplicate, *please* point to the earlier answer (because I've searched and couldn't find it myself).

PPS: It's now September, and I'm growing accustomed to the default (two screens). Other display settings persist, but the mirroring never did.