Follow the metal walkway until you can’t go any further. Take a look around as you watch the next test being built right in front of you! Once you’re in the room proper, turn left and pick up the Displacement Cube. Now angle it so the laser beam is passing through both of the ‘cones’ at the same time.

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Now lay your first portal at the point where the beam touches the wall and then position the second portal so it’s just to the left of the untouched ‘cone’.

The beam should now pass through all three cones at once, opening up the exit to the next test.