After obtaining the Missile Tank, head back up the hidden passage and open the red blast door to your right. Continue through this door and proceed right. You'll enter a large, open area with several platforms and enemies. Navigate this area, generally heading upwards and to the right. You'll eventually reach a room with a large, glowing orb. This is the Long Beam upgrade.
Collecting the Long Beam will significantly increase the range of your Power Beam, making it much easier to hit distant enemies and destroy blocks from afar. This upgrade is crucial for efficient combat and exploration. With the Long Beam and Missiles, you're now much better equipped to handle the challenges of Brinstar and beyond. Continue exploring Brinstar, looking for more Missile Tanks and Energy Tanks to bolster your resources.