The answer is 5.33 x 10^-4
to find the answer follow these steps:
the reaction is first order reaction
 here k = 6.29 x 10^-4 s^-1
 initial concentration = 1.00 x 10^-3
 After time t = 1.000 x 10^3 s concentration is = ?
 So, - 6.29 x 10^-4 x 1.00 x 10^3 + ln (1.00 x 10^-3) = - 7.54 
 C = e ^ - 7.54= 5.33 x 10^-4 M