=Interval(TimeStamp#(SOLVED_DATE_TIME) - TimeStamp#(START_DATE_TIME),'mm:ss')
which will give you the minutes and seconds.
If you just want minutes then:
=24*60*(TimeStamp#(SOLVED_DATE_TIME) - TimeStamp#(START_DATE_TIME)
Will give the minute which if you can round if needed.
I think it would be good to read about how you can help Qlik Sense read your date fields correctly
Once you get your dates read correctly into Qlik Sense, you just need to use Interval function to get the values in Minutes