if statement with an underline

[kenpcli]

How do I write an if statement that says if cell d10 doesn't match cell d9, then underline the entire row, then go to the next cell until there is nothing in a cell.

 

[Michael M]

You can't do that by formula.
You will either need Conditional Formatting or VBA
IF using CF, do it this way

Home tab>>Styles>>Conditional Formatting>>New Rule>>select "Use formula to determine">>
In the formula box type =D10<>D9>> Format as required>>OK