Leviton 8299 combination switch and receptacle with GFCI manualhttp://waterheatertimer.org/pdf/Leviton-8299-instruction-manual.pdf
You don't say what kind of device you are replacing.
Instructions below may be way off base if you are trying to replace a 3-way switch.
Instructions below are for replacing a switch-plug combo.
Add a comment any time, and clarify your device your are replacing.
Electricians don't guess, they test.
First thing we need to test is Hot and Neutral wires.
Note where each wire was connected to old device before removal.
Note the wire that was connected to brass, dark, and silver screws.
The screw color is coded, and will verify testing.
Separate wires and turn on power.
Test each wire to bare ground wire.
Tester will light up on Hot wire. Mark the hot wire.
Test Hot wire to each other wire (except bare ground wire)
Tester will light up on Neutral wire. Mark the Neutral.
The other two wires are presumed to go to Load (light, fan, motor)
Now lets look at back of the 8299 device.
On back of device directly behind receptacle part of device is the word LINE.
Hot wire connects to brass colored LINE screw.
Neutral wire connects to silver colored LINE screw.
These two wires will power the device, and power the receptacle.
Next we want to wire the switch.
Notice on drawing below, that switch Loads (light fan motor) are connected to two wires on back of 8299.
Again referring to back of device as reference point, black wire going to Load connects to wire on left side. White wire going to Load connects to wire on right side.
Now turn on power and test that switch and receptacle both operate as expected.
If operation is not correct, then add a comment and say what the second black and white wires connect to.