Firstly rock is thrown in upward direction with initial velocity u= 15m/s. As the rock reaches top most point, it's final velocity (v) becomes zero. Therefore height reached by rock during this interval can be obtained by kinematic equation since acceleration is constant. Therefore, v^2 - u ^2 = 2ah V=0, u= 15m/s, a=-9. 8m/s^2 ( minus since acceleration is in opposite direction or we can say rock experiences deacceleration). Putting these values in above equation we get h=11.48 m. Now rock falls from this point with initial velocity (u= 0). a= 9.8 (since rock will experience acceleration), h= 11.48 +50 = 61.48 m. Again using the same above given equation : v^2 - u ^2 =2ah. V^2 - 0 = 2 x 9.8 x 61.48