In order to slide a heavy desk across the floor at constant speed in a straight line, you have to exert a horizontal force of 800 newtons. compare the 800-newton horizontal pushing force f to the frictional force f between the desk and the ground.

The hint is "constant speed". 
If there is a net force on an object, what occurs to that object? F net = m*a It accelerates! 
So, if the desk is going at a CONSTANT speed, what does that tell you about the acceleration? There is a change in speed. 
Fnet = m*a Since a = 0 Fnet = 0 
Fnet = sum of the force... reflect the direction of F as the positive direction. Sum of the forces in the direction of F = 0 0 = F - friction F = friction 
So, if F = 800 N, then friction = 800 N in the opposite direction.