Make a diagram showing forces acting on a coasting bike rider traveling at 25km/h on a flat roadway

If he's coasting on a flat road, then there is no acceleration ...

his speed is constant and he's moving in a straight line.

No acceleration means the net force acting on him is zero.

Horizontal forces add to zero:

Nothing pushing him from behind.

Nothing pulling him from in front.

No horizontal forces.

Vertical forces add to zero:

Force of gravity pulling him down, equal to the weight of him plus the bike.

Equal normal force, pushing him up, exerted by the road.