The point of this room is to make the laser light shine through all three lights on the floor. It's already shining through one of them. Be careful because the laser will kill you if you stand in it.

First, move the laser redirection cube out of the path of the laser. The laser will shoot the wall across from it. Put a portal on the wall where the laser is shooting. Then put a portal of the other color on a wall such that the laser will go through one of the target lights, preferably one of the two that is next to the far wall.

Now pick up the redirection cube and jump over lasers. You want to put the redirection cube into the path of the laser that has gone through a portal. In other words, don't block the laser before it goes through the first portal. Using the cube on the second half of the laser, aim the laser straight through the two remaining target lights, being careful not to get hit by the laser.

Go through the exit door.