Physics help please?

The 20N upward force comes from Newton's 3rd law of motion. Every action (force) has an equal and opposite reaction (force). Therefore, the 20N downward force from the book due to gravity is countered by an equal upward force by the table (normal force). Otherwise, there would be a force imbalance meaning that the book would have to be accelerating in some direction. For uniform motion (in our case v = 0 since the book is static), F = 0. Thus, the sum of all forces must equal zero.