think syntax error..
try this
Declare @SQLString1 NVArchar(Max),@SQLString21 NVArchar(Max),@SQLString3 NVArchar(Max) , @SQLSTring nvarchar(max)
declare @db_name varchar(20); set @db_name = 'master';
SET @SQLString21=' where type=''U'''
Set @SQLString1='Select * from sysobjects'
SET @SQLString3=@SQLString1+@SQLString21
SET @SQLString = N'USE ' +...