Challenge 1: Cutting a Circular Notch Out of a Rounded Rectangle's Corner — Solution Walkthrough

The answer:

Subtract (labeled "Minus Front" in Illustrator, "Difference" in
Inkscape) is the right operation here. Per this chapter's own
material, it cuts the overlapping area of the front shape out of the
back shape, like a cookie cutter.

How to actually build this from Chapter 4's basic shapes:

Draw the rounded rectangle first (Chapter 4's Rectangle tool), then
draw a circle (Chapter 4's Ellipse tool, held with Shift for a perfect
circle) positioned so it overlaps the corner where the notch should
be. With the circle placed in front of the rectangle, applying
Subtract/Difference removes the circle's overlapping area from the
rectangle, leaving a rounded notch cut into that corner.

Why this is easier than drawing the notch by hand:

Per this chapter's own opening framing, this exact kind of shape — a
rounded rectangle with a circular notch — is far easier to build from
two simple, overlapping shapes than to hand-draw with the Pen tool
from Chapter 3, which would require manually curving a notch shape
into an otherwise straight-edged rectangle.

WHY THIS WORKS AS AN ANSWER
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This exercise checks that a cut-out notch is correctly matched to the
Subtract/Difference operation, and that the front-shape/back-shape
relationship (which shape gets removed from which) is understood
correctly.