Portal Chapter 9 (Puzzle 17)
As you enter this level you’ll be given a ‘companion cube’. You must take this cube with you to the very end of the level as you’ll need it multiple times. Place the cube next to the step on the right. Jump onto the cube and then onto the first ledge. Pick up the cube and place it down next to you so you can get up the second step. Turn around and pick up the cube.
Hold the cube in front of you as you run down the next corridor to avoid being killed by the light balls. Go up the second corridor. This time walking backwards to protect against light balls from behind. You’ll come to a room with three pathways on the floor. Drop down with the cube.
Each of the pathways connects to a platform that raises once it’s activated. Head into the dark room on the left. There’s a light ball being thrown against a wall. Use the companion cube to deflect the light ball into the receptacle. It may take a few tries. Once that’s done the first platform will raise up.
Use portals to get back up to the ledge where you came from. Place a portal on the sloped tile opposite the receptacle, as shown in the picture below.
Go back to the corridor and place the second portal on the wall where the light balls are hitting. The ligth balls will go through the portals and hit the raised receptacle which activates the second platform.
Drop back down into the room with the lines and pick up the companion cube. Take it into the room with the button. There’s also a button in the far left corner on top of a block. Drop the cube in front of it so you can jump up to it. Pick up the cube and drop it on the red button. This opens the door next to it with a receptacle.
Leave the room and use portals to get back up to where you came from. Go back through the first corridor and then place a portal at the end of the second corridor where the light balls hit. You’ll need to do this quickly or you’ll get hit from behind.
Go back to the lowered area and stand on the red button just inside the room with the receptacle. Place the second portal on the wall opposite. Duck when the light ball starts coming towards you and let it the receptacle behind you.
This will raise the third platform. Grab the companion cube from the button in the corner and portal your way up to the higher ledge. Jump across the three platforms and place the cube on a red button which opens a door to another button that opens the incinerator.
Press the little red button and then quickly grab the companion cube and throw it in. This opens the door to the level exit.