Explanation:
First simplify any radical. This is only allowed with √12 since it's the only one that can be simplified. √12 is equivalent to 6. Now you have the expression √3(6+√5). You can't simply the remaining 2 radicals. You also can't multiple a radical and a constant. BUT you can combine √3 and √5. You multiple 5 and 3 as that's that it's telling you to do. Thus, leaving you with 6 + √15. Having a radical and a constant, this is the simplest you can go as you can't combine these 2 together.