Answer:
![b. \ y=3cos(3\theta)](https://img.qammunity.org/2020/formulas/mathematics/middle-school/cbfvc2kx4mzcv7ujgyyw2rcdbva65q5q84.png)
Explanation:
The cosine function is a trigonometric function whose curve is symmetric with respect to the y-axis, hence this function is said to be even. This function has the following form:
![y=acos(b \theta)](https://img.qammunity.org/2020/formulas/mathematics/middle-school/iq0dukrmqvitb11277zigdcypbax4ph711.png)
The amplitude of the plotted graph is 3 that represents half the distance between the maximum and minimum values. Since at x = 0, y = 3, this y-value is the value of a. On the other hand, the period, which is the length of one cycle of the curve, can be found as
and:
![T=(2\pi)/(b) \therefore b=(2\pi)/(T)=(2\pi * 3)/(2\pi)=3](https://img.qammunity.org/2020/formulas/mathematics/middle-school/g3rwmupfmouayi8nf5zsjs327pk1gschay.png)
Finally:
![\boxed{y=3cos(3\theta)}](https://img.qammunity.org/2020/formulas/mathematics/middle-school/rfr1qintjao54n4jis294ucsi75pap5s4x.png)