The mass of the piano is 185 kg, therefore its weight is
W = (185 kg)*(9.8 m/s²) = 1813 N
The normal reaction force on the piano from the floor is N = 1813 N, according to Newton's 3rd Law.
The coefficient of friction between the piano and the floor is given as μ = 0.39.
By definition, the frictional force is
F = μN
= 0.39*1813
= 707.07 N
Answer: 707 N
