A 2Kg book rests on a table.  What force is applied by the table on the book?

Force is in Newtons, but we are given Kilograms.  We must switch to Newtons.  How?  Realize that the acceleration present is due to gravity, 9.8m/s/s

Force of book downward = ma = 2Kg * 9.8m/s/s = 19.6 Newtons

The situation is in equilibrium (no net force) so according to the 3rd law, the table's force upward must also be 19.6N

An 85 Kg teacher rides up in an elevator that accelerates at 1m/s/s.
What would a scale read if the teacher were to stand on it?

F = mg = 85Kg * ( 9.8m/s/s + 1 m/s/s) =  918N
We had to add the acceleration of the elevator+gravity because in effect, they are acting together.

What would it read if acceleration were -1m/s/s?

F= mg = 85Kg * ( 9.8m/s/s - 1m/s/s) = 748N

What if the elevator stopped accelerating and moved at a constant speed?

There is no acceleration, so we know the forces are balanced.  This means you would weigh what you would weigh on a scale sitting on solid ground...
F = mg = 85Kg * 9.8m/s/s = 833N
now you should know why you only feel funny at the beginning and end of the ride.

What is the mass of the teacher?

Mass is merely the amount of stuff in an object, and does not depend on gravity, so it is just
85Kg