A stone is trown vertically up from a bridge with velocity 3$m{s}^{-1^{ }}$. If it strikes the water under the bridge after 2 s, the bridge is at a height of $(g=10 m{s}^{-2})$

According to question,

The Initial Speed of stone = u = 3m/sec.

Time , t = 2 sec,  g = 10m/sec²

Equation of motion = S = ut + $\frac{1}{2}a{t}^2$

As we are calculating the height, velocity is acting downwards so, 

h = ( -3 x 2 ) + $\frac{1}{2}\times 10\times 2^2$ 

h = -6 + 20

h = 14m.