Answer: 
void calculatеAvеragе(double array1[],int count);//function declaration 
Explanation: 
Here we are declared a function calculatеAvеragе() of void return type because there is no return type is mention in the question so I take void return type. The calculatеAvеragе() function takes two parameters one is an array of type double and other is a variable count of int types. 
To declared any function we used following syntax 
return_type functionname(parameter 1,parameter 2 .....parameter n) 
So void calculatеAvеragе(double array1[],int count);