You cannot define variables in a query. You need to use declare statements and define variables before using them
Try this instead
select @UnderPartA as UnderPartA,@UnderPartB as UnderPartB, (UnderPartA + UnderPartB) as TotalDEntist
Alternately,
Declare @UnderPartA INT,
Declare @UnderPartB INT,
Declare @SumTot INT
Declare @DentalHysenist INT
select @UnderPartA as UnderPartA,@UnderPartB as UnderPartB,@DentalHysenist as DentalHysenist,@DentalMachenist as DentalMachenist,
@SumTot = @UnderPartA + @UnderPartB;
PRINT @SumTot