By what distance, x2, should the second child compress the spring so that the marble lands in the middle of the box? (assume that height of the box is negligible, so that there is no chance that the marble will hit the side of the box before it lands in the bottom.)
express the distance in terms of m, k, g, h, and d.
there are many different formulas you can use, but the simplified form that mastering physics is looking for is this one:
x2 = d / sqrt(2kh / mg)
x2 = d / sqrt(2kh / mg)