Easy-Going Genius

During the main story mission Driver’s License Lunacy, once you're playing as Goku and have Piccolo/Gohan as support, you'll need to head to Goku's house. Before you do that, make a detour to Capsule Corporation. You'll find Dr. Briefs there with a new substory for you. He needs you to gather some materials for a new invention: a Skull Robo Gear, some Densite, and Silver Ore. He'll mention that Densite can be found by breaking large rocks.

After accepting the substory, exit Capsule Corporation and head west from the city. Your objective marker will lead you to the entrance of a small cave near the water. As you get close, you'll be ambushed by some robots. These are the same Skull Robos you've fought before, so they shouldn't pose much of a challenge. Defeat all of them, and you'll automatically obtain the Skull Robo Gear. If you happen to be short on Silver Ore, the same cave area has nodes you can mine for it.

Now, head to the next marker on your map. Dr. Briefs mentioned that Densite is found by breaking large rocks. Look for the biggest boulder you can find in the area indicated by the marker. You can destroy these by using your Ki blasts or by flying directly into them. Once you destroy the boulder, search the rubble for a familiar purple orb. Collecting this orb will give you the Densite.

With all the required materials in hand, return to Dr. Briefs at Capsule Corporation to turn them in and complete the Easy-Going Genius substory.