The correct answer is Choice C.
Move all the other terms to one side to isolate the term, $a$, of interest.
Choice A is incorrect and may result from subtracting 3 instead of dividing 3 from both sides.
Choice B is incorrect and may result from a sign error when isolating $a$.
Choice D is incorrect and may result from multiplying 3 instead of dividing 3 from both sides.
✨ Expert's Tip ✨
To solve for three variables, three equations are required. In general, to find $n$ variables, $n$ equations are required.