Exercise 1: Setting a Headless Server's Default Target — Possible Solution
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sudo systemctl set-default multi-user.target

-- Why a headless server should never default to graphical.target --
--
-- Per the chapter's own compare-table, graphical.target is
-- multi-user.target plus a display manager/GUI on top -- and per the
-- chapter's own description, "graphical.target itself Wants=
-- multi-user.target, since graphical mode is a strict superset."
-- A headless server, by definition, has no display attached and no
-- one physically present to interact with a graphical desktop
-- environment at all. Defaulting to graphical.target on such a
-- machine would mean systemd spends real boot time and system
-- resources starting an entire display manager and desktop stack
-- that literally cannot be used or seen by anyone -- pure waste, with
-- zero functional benefit, since there's no monitor for it to display
-- on and no one there to log into it.
--
-- multi-user.target provides everything the server actually needs --
-- full multi-user networking, services, remote access via SSH -- without
-- the unnecessary overhead of a graphical stack the hardware isn't even
-- equipped to display. This is exactly why the chapter names
-- multi-user.target as the typical default for a headless server, and
-- graphical.target as the typical default only for a desktop distro
-- where a physical display and user are actually expected to be
-- present.

WHY THIS WORKS AS AN ANSWER
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This gives the correct command using the chapter's own exact syntax,
then explains the reasoning specifically in terms of graphical.target
being a strict superset of multi-user.target (per the chapter's own
stated relationship) and why that extra layer is pure waste on
hardware with no display.