finding a trig equation from graph

I have a graph, the maximum value is 36, the minimum value is 4 and it cycles every 6 hours. So at time = 0 the graph is at 4 on the y axis, then at time 6 it is at 36, 12 it is at 4 hours, and so on. So the points are (0,4), (6,36), (12,4) etc. I have to find the equation for this graph using a transformation of y = sin(x). So far I have y = 32sin(π/9 t) + 4, but everytime i run this on a graphing simulator the results go into the negative values so i am sure it is wrong I'm really stuck here, and it is quite frustrating, can anyone help. thank you.