If the initial concentration of xy is 0.050 m, what is the concentration of xy after 55.0 s ?

Missing question: The decomposition of  XY is second order in and has a rate constant of 6.96×10⁻³ s⁻¹ at a certain temperature.
c₀ = 0,050 M.
t = 55 s.
k = 6.96×10⁻³ s⁻¹.
Integrated second order rate law is: 1/c₁ = 1/c₀ + k·t.
1/c₁ = 1/0,050 M + 0,0069 s⁻¹ · 55 s.
1/c₁ = 20,38 1/M.
c₁ = 0,049 M.

Ido not understand.