Try something like this...
DECLARE @result float;
DECLARE @Diam NVARCHAR(50);
DECLARE @S NVARCHAR(max);
SET @Diam = '5-(22/5)';
SET @S = N'SELECT @result = (' + @Diam + ')';
PRINT @S
exec sp_executesql @S
, N'@result float OUTPUT'
, @result OUTPUT
select @result