Second Law Equation

[Gerald Bernal1I]

Is the equation only used to find entropy of the surroundings or is it used to find the entropy of the system?

[christabellej 1F]

It is used to find the entropy of both and can be used for everything because entropy is a state function (so it doesn't matter how you get there, and thus how you calculate it). If you are finding entropy of the surroundings though under constant pressure, entropy of the surroundings would be opposite of the entropy of the system. For example, if entropy of the system increased by 2 J/K, then the entropy of the surroundings under reversible conditions would have decreased by 2 J/K.