Hi there, doing some 11th grade trig I've run into difficulty. Here are the questions I'm having trouble with:

(a) Evaluate h(t) = cos (20t)° for t = 3.

(b) What is the value of t when h(t) = 0.3 for 0 ≤ t ≤ 18?

Answers from the book's appendix:

(a) 1/2

(b) 3.6, 14.4

For (a), I get 1/2 no problem, which the book lists as the correct answer. But for (b), I only understand how to get 3.6 and not 14.4 - conceptually and practically, I'm not following how to get the 14.4 beyond a vague idea that this involves periodic values of the function.

If you could point me in the right direction or give me a similar problem with the solution and steps for that problem, I'd really appreciate it.

Thanks