Its a seconds pendulum ( not necessary to mention it actually ). You have, l = 4π²aT².
Where, l → length of pendulum.
a→ acceleration. T→ time period.
Initially, a=g=9.8, now it decreases by 2.8 then a=7.8
Apply the above relation, you get new time period = T(7/(√39).
But if you approximate g≈10, you it as T(5√2/6).