How did they get this math answer? easy 10 points :)?

To find the the time you just divide by gravity 24.5 / 9.8 = 2.5 seconds, you can even check the units, this is correct . For the second part you just need to find when the velocity equals zero, in this case it's half the time it took to reach its max height, 1.25 * 24.5 = 30.625, but dont forget that you started at a height of 12m, so you just add that on giving 42.625 or 42.3m