You take off in a spaceship traveling at an unknown relativistic speed. Your twin sister of the same age (25) stays on Earth. You travel at a constant speed during the trip and have the unfortunate coincidence of arriving back on Earth on the exact day your sister passes away at the age of 75. Looking at the calendar on your ship, you are now 34 years old.
What was your velocity (relative to the speed of light, $c$) on this trip?