I have asked this question before but i'm still not clear on how the answers were derived and when I do it I get a different answer so i'm attempting for a 3rd time

A boy wants to find the initial speed with which he can throw a stone. He found that he can throw an object vertically and barely reach the roof of the building where he leaves. He used a dense spherical object to make the air resistance negligible. The distance from his extended hand to the roof is 15 m.
What is the initial speed of the projectile in m/s, km/h, and miles/s ?
Use the formulas:
V=V0-gt ,
X=V0t-(1/2)gt^2 .

(Note: This experiment was done with an object that would not break a window but is round and dense, like raw apple or better raw walnut.)