Answer
The stereocenter of this molecule is the carbon that is making bonds with an ethyl and a methyl. (The one with 3 visible bonds on the right.)
Work Step by Step
1. Look for all the carbons that are making at least 3 bonds.
** If a carbon has less than 3 visible bonds, it means that it has 2 hydrogens (equal groups) or has a double bond. Therefore, it can't have 4 different groups, so it can't be the stereocenter.
2. Since we know that there is only 1 stereocenter, find the carbon that has 4 different groups connected to it.
** Carbons with double bonds can't have 4 different groups.
** Don't forgot the Hydrogen.
If a carbon follows this rule, it is a stereocenter.