Month by month comparison

Create 2 date tables, one for each slicer. Do not link these tables to your main fact table. You can then produce measures like

Month 1 Value =
CALCULATE (
    SUM ( 'Table'[Risk Factor] ),
    TREATAS ( VALUES ( 'Date Slicer 1'[Date] ), 'Table'[Reporting Date] )
)
Month 2 Value =
CALCULATE (
    SUM ( 'Table'[Risk Factor] ),
    TREATAS ( VALUES ( 'Date Slicer 2'[Date] ), 'Table'[Reporting Date] )
)

To allow users to select any two ReportingDates and compare the RiskFactor for each SupplierSiteID, the best approach is to use two disconnected date slicers combined with DAX measures.
1. Use following DAX for both date slicers -

DateSelection = DISTINCT(SELECTCOLUMNS('YourData', "ReportingDate", 'YourData'[ReportingDate]))

OR 

DateSelection = CALENDAR(MIN('YourData'[ReportingDate]), MAX('YourData'[ReportingDate]))

Use the DateSelection[ReportingDate] field twice - once for each slicer. Label them “Date 1” and “Date 2” to make the user selection intuitive.
2. Create Measures for Selected Dates

SelectedDate1 = MIN('DateSelection'[ReportingDate]) -- for slicer 1
SelectedDate2 = MAX('DateSelection'[ReportingDate]) -- for slicer 2

3. Create Measures to Fetch the RiskFactor

RiskFactor_Date1 = 
CALCULATE(
    MAX('YourData'[RiskFactor]),
    'YourData'[ReportingDate] = [SelectedDate1]
)

RiskFactor_Date2 = 
CALCULATE(
    MAX('YourData'[RiskFactor]),
    'YourData'[ReportingDate] = [SelectedDate2]
)

4. Compare the Values

RiskFactor_Diff = [RiskFactor_Date2] - [RiskFactor_Date1]

You may wanna use percentage here to show your result. 

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If this solution worked for you, kindly mark it as Accept as Solution. This would be helpful for other members who may encounter similar issues and feel free to give a Kudos, it would be much appreciated!

Thank you,
Sarita