The answer is a.
When you are solving problems like this you would use the equation y^2=4ax. The x and Y would be the coordinates. Based on the equation you have given the coordinates would be (6,12) and (6,-12), (24 is split up evenly between the y-axis).
Then we go on to solve the equation by plugging in the numbers 12^2=4a(6)
144=24a
144/24=24a/24
6=a
