Samantha pulls a wagon with her dog in it. the mass of the wagon and the dog together is 45kg. the wagon accelerates at a 0.85m/s2. what force is molly pulling the wagon with.

There's really no way to calculate Molly's pulling force, because all
of the data given in the question only describes Samantha's situation. 

The following solution assumes that Molly's wagon and dog are identical
in mass, acceleration, and frictionlessness to Samantha's: 

Force = (mass) x (acceleration) =

             (45 kg) x (0.85 m/s²) =  38.25 newtons

(Assuming there's no friction anywhere in the wagon/ground system.)