k = {2.303/t} log [P_o(y-1) / yP_o - P_t ]

here k = rate constant

P_o = initial pressure

P_t = pressure at that instant

y = no. of products

t = time

substitute the given values u get the answer as

k = 2.175 * 10^-3 s^-1

see this formula holds good for first order rxns...