Wait a moment for the lasers to activate and some platforms to rise. To open the exit, you need to make the two lasers go into to the two laser receivers. However, you can only create two portals, which means that you can only use portals to make one of the lasers go through one of the receivers. For the other, you will have to use the discouragement redirection cube.

Use a portal to reach the ledge where you can see the discouragement redirection cube. While up here, create a portal on the red spot on the floor on this ledge. Then create a portal of the other color on the ceiling above the raised platforms.

Pick up the laser redirection cube and drop through the floor portal so that you and the cube land on the raised platforms. Hold onto the cube and put it into the path of the laser that passes over the platform, and aim it so it shoots into the laser receiver on the wall nearby.

Take a look at the portal on the ceiling above you. Create a portal of that same color on the wall where the other laser is shooting. This should make the laser shoot into the receiver on the ceiling where you picked up the redirection cube, opening the exit. Go through the exit and into the elevator.