As we know that time period of simple pendulum is given as
T = 2π √L/g
here we know that
T = 3.8 s
now from above equation we know that
T² = 4π² (L/g)
now on rearranging the above equation we will have
L = gT² / 4π²
now plug in all data into it
L = (9.8) (3.8)² / (4) (3.14)²
so the length of the cable must be 3.6 m