How many carbon atoms are found in a molecule of 2-methyl-3-nonene? 8 9 10 11?

The correct answer is 10. The nonene is the base chain, with a double bond between the 3-4 carbon. The non prefix dictates there are 9 carbons in the chain. The 2-methyl tells you that there is a methyl group coming off of the second Carbon in the chain. Methyl is CH3. Therefore your answer is 10.