Find Amplitude, Midline, and Period/Frequency of Periodic Graphs

Find a possible formula for the trigonometric function whose values are given in the following table. $\\\begin{array}{|r|c|c|} \hline x&0&3&6&9&12&15&18\\ \hline y&-4&-1&-2&-1&-4&-1&2\\ \hline \end{array}$

This can be expressed with a cosine or sine function, so we will use the cosine function.