Here's a shot in the dark:

USE NORTHWIND

SELECT

 CustomerID

 , COUNT(*) No_of_Orders

 , DATEDIFF(d,MIN(OrderDate), MAX(OrderDate)) TimeSpan

 , COUNT(*)*1.0 / DATEDIFF(d,MIN(OrderDate), MAX(OrderDate)) DailyOrder

FROM

 Orders

WHERE CustomerID = 'ERNSH'

AND (OrderDate >='19960717' AND OrderDate <='19960723')

GROUP BY CustomerID

Btw, since this is an exercise given to you, don't you think you should find the solution yourself?