P = W/t = FxS/t = m x s x a /t

Now, P1 = P2 (since P is constant, given in question)

=> m x a1 x s /t = m x a2 x s /t

or a1 = a2

i.e., acceleration is constant.

=> equations of motion may be applied.

so,

2as = v^2 - u^2

i.e., 504a = 36 - 9

or a = 27/504.

v = u + at

v - u = at

t = (v - u) / a

= 3 x 507 / 27 = 56s