Sinusodial graph help (electrical)

Solution:
The wave equation can be of the form:
$$y=A\cos(B(t-C))+D$$
where $A=4$ is the vertical max distance from centerline of symmetry which is called amplitude. D is vertical shift here which is clearly $D=-2$
C is phase shift which is two boxes left. So, from graph it is clear $C=-\frac\pi 4$
The wavelength (period here) of the graph is approximately 15 boxes. So it is defined by $$\lambda=\frac{15\pi}{8}$$
And B is defined as wave number given by
$$\frac{2\pi}{B}=\lambda$$
which implies $B=\frac{16}{15}$
Thus, we can answer all the parts as:

a) Amplitude is $4$, periodic time is $\frac{15\pi}{8}$, phase angle in degrees is $\frac{4\pi}{15}\times \frac{180}{\pi}=48$ degrees, vertical displacement is $-2$ and frequency is reciprocal of period which is $\frac{8}{15\pi}$
b) The cosine function is given by $$f(t)=4\cos\left(\frac{16t}{15}+\frac{4\pi}{15}\right)-2$$
c) By using the fact that $\sin x=\cos(\pi/2-x)$ we get the sine function as
$$f(t)=4\sin\left(\frac{7\pi}{30}-\frac{16t}{15}\right)-2$$