Create a Identity column in your table and append that with your Fixed character RENT
as below
CREATE TABLE t1
(
c1 int identity(1,1),
c2 char(3)
);
GO
INSERT INTO t1 VALUES ('2'), ('37'),('597');
GO
SELECT 'RENT' +REPLICATE('0', 3 - LEN(c1)) + convert(varchar(10),c1) AS 'Varchar Column'
FROM t1;