The period of a simple pendulum is given by:

where L is the pendulum length, and g is the gravitational acceleration of the planet. Re-arranging the formula, we get:

(1)
We already know the length of the pendulum, L=1.38 m, however we need to find its period of oscillation.
We know it makes N=441 oscillations in t=1090 s, therefore its frequency is

And its period is the reciprocal of its frequency:

So now we can use eq.(1) to find the gravitational acceleration of the planet:
Answer:
6.49 x 10^-8 N
Explanation:
formula is
F= G * ((m1 * m2)/r^2)
F = 6.67x10^-11 * ((6.8*6.8/.218)
F = 6.49 x 10^-8 Newtons
The upward force the table exerts on the ground!
Equal and opposite forces.