The period of the argument is 2π, because the cosine starts at 1 when the argument is zero and returns to this point when the argument is 2π. The frequency is 1 cycle in the period.
Comparing cos(2x) with cos(x) we can see that the function produces 2 cycles where before there was only one. A phase shift in the argument (the constant term) does not affect the frequency, so since the coefficient of t is the same for both functions the frequency is 2.85.