What is the total retarding force

A kid on a tricycle is being pushed by her father at a constant velocity.The father exerts a
constant force of 10.0 N at an angle of 30.0o below the horizontal. If the combined mass of the kid and the tricycle is 30.0 kg, what is the total retarding force (e.g. air friction, friction on the
tires, etc.) acting on the kid and the tricycle

Horizontal force = 10 . cos 30 = 8.66 N

Since the trike is going at constant speed, the retarding forces must be the same as the pushing force.... the retarding forces must be 8.66 N