2. No, this is a shorthand for "Solutions are x = 30, x = 150, and x = 270." Writing "x = {30, 150, 270}" would be a type mismatch because x is a number (presumably, a solution to an equation), while the right-hand side is a set of numbers. One could write, "The set of solutions is {30, 150, 270}," though this sounds too pedantic. Another way is, "x is a solution iff $x\in\{30, 150, 270\}$."