In your microflow:
	- create an Integer variable called WeekCount
- use the following expression to set that variable  parseInteger(substring(ColumnB,find(ColumnB,’ ‘)-1))
- Set the value of ColumnC with the following expression addWeeks(ColumnA,-1*$WeekCount)
 
Hope that helps you get started.
**EDIT**
my expression was wrong.  I think it should be:  parseInteger(substring(ColumnB,0,find(ColumnB,’ ‘)))