Answer:
0.999958c
Step-by-step explanation:
Remember that the trip involves traveling for six months at a constant velocity thus returning home at same speed . The time interval of this trip will therefore be one year.
Accurate time interval to be measured by the clock on the spaceship Δt0 = 1.0 years
Time interval as advanced on earth observed from the spacecraft Δt = 110 years
the formula for time dilation is
 Δt = Δt0 /√(1-v2/c2)
 or v = c*√1-(Δt0/Δt)2
 =c*√[1-(1year/110years)2]
 = 0.999958c