Joint variation means that y=Axz, where A is the constant of variation. To find A plug in the values given:
72=12A, so A=6 and y=6xz. Now plug in the other values of x and z to find y: y=6*4*12=288.
Another way: y=72 when x is 6, so when x=4, forgetting about z for the moment, y would be smaller by the factor 4/6 or 2/3, making y=48. However, z is 6 times what it was before (12/2=6) so y would be bigger by a factor of 6, and y=6*48=288.