How can I use variables with "LineString"

I keep getting this error message every time I run this code:

DELIMITER $$

CREATE PROCEDURE BBOX(IN X FLOAT, IN Y FLOAT, IN E FLOAT)

BEGIN

DECLARE A, B, C, D , R FLOAT;

SET R = E/69;

SET A = X - R;

SET B = Y + R;

SET C = X + R;

SET D = Y - R;

SELECT NAME, ASTEXT(LOCATION) FROM POINTS WHERE MBRContains(GeomFromText('LINESTRING(A B,C D)'),LOCATION);

END $$

DELIMITER ;

ERROR 3037 (22023): Invalid GIS data provided to function st_geometryfromtext.

I'm trying to create a Bounding Box procedure that searches for points in my database that are an entered number of miles apart from an starting point.