As per Newton's law we know that
F = ma
here we know that
F = 4 N
m = 2 kg
so from this equation we will have
4 = 2 a
a = 4/2 = 2 m/s^2
so the acceleration will be 2 m/s^2
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
