Here it says determine the number of hydrogen atoms attached to each of the circled carbon atoms. All right, so we know that every line represents a connection between 2 carbons. And remember, carbon must make four bonds. So the carbon on the left, we see it making one bond to this carbon. In the middle. It needs three more bonds, which would mean that it has three hydrogens that we don't see.

Now, the carbon in the middle. Let's go to this one. We see it making what? 123 bonds. Carbon must make four bonds. So it's one bond short, which means it has one hydrogen. That's invisible. And then finally the carbon on the right. We see this carbon here making 1-2 bonds. Again, carbon must make how many bonds? 4. So it's missing two more bonds, which means it has two hydrogens that are invisible.

So here we'd have 3 hydrogens, 1 hydrogens, and then two hydrogens going from left to right. So this would be the number of H atoms on each one of these circled carbon atoms.