804 n of force are applied to a 51.7 kg. What is the acceleration that the object experiences?

We can use Newton II here (where F=m*a), that F is the net (or resultant) force on the object, m is the mass of the object and a is the acceleration the object experiences.

This means, in this case there would be no friction and absolutely no other force which gives a component in the plane of motion, only then can you assume that F=804N.

Now using F = m*a

804 = 51.7*a

Therefore a = 804/51.7 = 15.55 m/s²