help with my physics homework please.?

Question

A ball is thrown upward from the ground with an initial speed of 31 m/s. At the same instant, a ball is dropped from rest from a building 15 m high. After how long will the balls be at the same height?
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I tried to graph the movment of the two objects and and used the point of intersection as the awnser, but the webassign program said i was wrong. Any help would be great.

Answer 1

Ball one is thrown upward and ball two is dropped

Ball one has

y1(t)=31*t-.5*g*t^2

ball 2 has
y2(t)=15-.5*g*t^2

when the y(t)s are equal, the balls are at the same height
so
31*t=15

t=15/31

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