There are several answers possible ......
Using the form y = A sin ( Bx + C) + D
The period of the graph is 15 units in length.....then each quarter point = 15/4 units
The amplitude of the graph = A = 4
The midline of the graph is y = 1 = D
We can solve for B thusly
pi/2 = (15/4)B
pi/ 2 * (4/15) = B
(2/15)pi = B
And we can find C thusly......note that we are shifting the normal cosine curve to the right
1 = sin [ (2/15)pi (15/4) - C] + 1 subtract 1 from both sides
0 = sin [ ( pi/2) - C ] → cC = pi /2
So....our function is
y = 4 sin [ (2/15)pi* x - pi/2 ] + 1
Here's the graph : https://www.desmos.com/calculator/gdqmefwhyc
