How to change the driver's Expression to "var * 3 + var * 2"? I want to make a driver that combines the value of ShapeKey1 multiplied by 3 and the value of ShapeKey2 multiplied by 2.

I set the DriverSettings Type to Sum Values as shown in the image below. 

And, I entered "var * 3" and "var * 2" in Expression. 



However, an error message like the following image appears. 



What should I do?
Change driver type  to scripted expression, (from sum variables) call your variables say sk1 and sk2 and make the expression

3 * sk1 + 2 * sk2 


or 

sum(3 * sk1, 2 * sk2)


Currently you are typing an expression var * 3  into the name field of the variable hence the error.

To do this with sum variables driver type would need to add the sk1 variable 3 times and the sk2 variable twice. (all with unique names)