As can be seen from the diagram, the initial phase of the particle is given by
ϕ==30∘6π
The equation of motion of the particle at its initial position at time t=0 can be written as
x(t)=rcosθ...(1)
where, θ is the angular displacement of the particle at time t=t.
Since, the position of the particle at a later instant of time always lags behind its initial position at time t=0, the equation of motion of the particle at any instant of time is given by
x(t)=rcos(θ+ϕ)...(2)
Substitute the values of the known parameters into equation (2) to obtain the required expression
x(t)=rcos(ωt+6π)
