The given function is
f(x) = 15Cosx + 5
The amplitude is 15. The graph undergoes a vertical shift of 5 units upwards. This means that the maximum value would be 15 + 5 and the minimum value would be - 15 + 5
We would plot the graph representing this function. It is shown below
The maximum value of the graph is the value of y at the top and the minimum value of the graph is the value of y at the bottom. Looking at the graph,
Maximum value = 20
Minimum value = - 10