count of distinct two columns in one table

[91asma2]

Hi all, 
I am trying to calculation the unique number of users who did 2 or more activities from two separate columns in one table. 
My data looks like this: 

Table  

user_id	activity_id
1	aa
2	aa2
3	aa3
4	aa4
5	aa5

First I want to get the distinct count of users and then filter those users that have done 2 or more activities.
I am trying something like this, but I am not getting the values right. 

VAR allData =
ADDCOLUMNS(
VALUES(Table[activity_id]),
"rowCount",
CALCULATE( COUNTROWS( Table) )
)
return
CALCULATE(
COUNTROWS( allData ),
filter (allData, [rowCount] >=7)
)

I appreciate all the help. 

[amitchandak]

@91asma2 , Try a measure like

countx(filter(summarize(Table, Table[user_id], "_1", count(Table[activity_id])),[_1]>=2),[user_id])

[91asma2]

Thanks a lot @amitchandak !
Can you explain to me this forumla, more walk me thought it? 

I am trying to validate the values. and I am trying to change the filter from 2 to 1 to 3 ...etc but the number doesnt change. 

so I am not sure its counting the right values. 

What I am trying to get the count of users who has 2 or more activities.