Is one object dropped from the top of the building,

and the other thrown up from the bottom of the building?

You should end up with two versions of the \(\displaystyle s=ut +1/2at^2\) equation

when do these two equations give the same value for *s *?

As Studiot points out, be careful with your sign convention

If the acceleration (g) is positive down, then __all__ your other values must also be positive down (and up is negative).

i.e the initial velocity of the thrown object is negative

Also be careful about how you define your zero point for distance:

If you **chose** the bottom of the building to be at 0m, then (using up as negative) the top of the building is at **-**30meters

or if you **chose** the top of the building to be at 0 meters, then the bottom of the building is at **+**30 meters

You could use down as negative and up as positive, (but then g = **-**9.81m/s^2 and the initial velocity is **+**10m/s)

it is a matter of personal preference, and will give the same answer (but with the opposite sign)