when the cardboard is at rest, the MAGNITUDE of the pair of forces acting on it are EQUAL and the DIRECTION are directly OPPOSITE each other..
When an object (cardboard) is at rest, only 2 forces are acting on it: Gravitational Force and Support Force.
The Gravitational Force pulls the object down, while its reaction force (Support Force) pushes it up.
Example: A book is at rest on a table. Gravity is pulling the book down to the ground, but the table that it's on is supporting it (pushing it up).
a d c b e f n k a a s f e d d s g g r e d