Pentax K-X Operating Manual page 59

When using the built-in flash with an external flash (AF540FGZ or
AF360FGZ) that is set to the Trailing Curtain Sync mode, the built-in flash
will also use this mode. Confirm that both flash units are fully charged
before shooting.
When using the built-in flash with an external flash that does not have a
wireless flash mode function such as AF200FG, attach the Hot Shoe
Adapter FG (optional) to the camera hot shoe and an Off-Camera Shoe
Adapter F (optional) to the bottom of the external flash, and connect these
with the Extension Cord F5P (optional) as shown in the illustration below.
The Off-Camera Shoe Adapter F can be mounted to your tripod using the
tripod screw.
Only a P-TTL auto flash can be used in combination with the built-in flash.
When combining with the built-in flash