i have used

           azoisopropane -------> hexane + nitrogen

at t = 0         P₀                           0                  0

at t =360      P₀ - p                     p                    p

P₀ = 35

total pressure at t =360 s = P₀ + p = 54

p =54 - 35 = 19

so P₀ - p = 16

now use K = ln( P₀ / P₀ - p ) / t