OK.
The equation is d=vt+½gt².
d is the height of the helicopter over the house=300m. g=9.8m/s².
t is the time the criminal must get away=10 seconds. t²=100.
So we plug in the known values:
300=10v+½(9.8)(100)=10v+490,
10v=300-490=-190, v=-19.
So the explosive must be projected against gravity (that is, upwards into the air) at a speed of at least 19m per second.