Question 873518
Usually, we list answers in increasing order, so I would answer {2,4} or maybe {2, 4}.
The order should not matter, because the solution set {4,2} is the same set as {2,4}.
 
However, if you have to input your answers to be graded by a computer, try to guess what answer is expected. If the programmer was good, all reasonable answers will be accepted. If it's a cheap program, strange things could happen, such as {2, 4} being an accepted answer, and {2,4} (without the space) graded as wrong.
Anyway, if there is a sensible human in charge of verifying the grade, the human should be able to over-rule the computer, so communicate with the human if your right answer is graded as wrong.