Let Time period =T
Mass of the bob = m
Acceleration due to gravity = g
Length of string = L
⇒a=0 ⇒ Time period of oscillation is independent of mass of the bob
b+c = 0 - + c =0
Giving values to a,b and c in first equation
The real expression for Time period is
Therefore time period of oscillation depends only on gravity and length of the string.
Not on mass of the bob.