(spoiler, highlight)<<use the first lens cube to aim the beam at the diagonal wall. Put a portal there and in the next room, then you can use the other lens cube to activate the platform.
In the top section, put one cube in the right alcove, next to the blue filter, pointing upwards. Put one portal on the right concrete wall. Use the other cube down below to point the laser at the ceiling and put the other portal where the laser hits the ceiling. (Make sure that the laser is a bit toward the brighter edge of the portal.) You might have to tweak it a bit, but if you got it right, you'll hear the exit door open.>>

Just use the lens cubes to aim the beam at the lower target, to raise the platform. You can use the same trick as with the previous platform: stand in front of the beam to get the platform back down, then quickly step onto it.

At the top, put the portal on the left wall and place the cubes in front of the left two arrows. The arrows indicate the direction for the cubes. (To rotate a cube up or down: balance it on the edge of one of the lower blocks, then let it fall down; it will hit the ground rotated 90 degrees.)