Elements of an AC expression
The expression says that this signal has three frequency components in it (50, 150 and 300Hz).

The fundamental frequency of this signal is given by the GCD of the frequency components in it (50Hz).

Harmonic components are the components whose frequencies are integer multiples of fundamental. (We have 3rd and 6th harmonic frequencies present in the signal here)

The instantaneous value of V can be obtained by substituting the given value (5.12ms) for t in the expression.