Just like when multiplying exponents, you're really just adding them together. This is the same case, when you're "dividing" the exponents, you're really just subtracting them.
2/3(6x+12) = -24 can be simplified to 4x+8 = -24 so the first answer is right and if you further simplify it by subtracting the 8 you get 4x = -32 Which is the other right answer.