A seismograph is a heavy weight that hangs so still it lags behind the shaking ground, tracing each passing wave as a wiggle. The instrument that began the science of earthquakes.
Every quake launches two waves at once: a fast pressure wave (P) and a slower shear wave (S). The P-wave keeps pulling ahead, so the gap between their arrivals grows steadily with distance — a built-in ruler to the quake.
One station's gap gives a distance but no direction, so the quake could lie anywhere on a circle. Cross the circles from three stations and they meet at a single point — the epicenter.
Real seismologists plot that gap on a Wadati diagram and pool dozens of stations worldwide to pin an epicenter within a kilometre or two — the very geometry you are using here.
Something in the simulation stopped unexpectedly — the lesson continues without it. Nothing you did was wrong; you can move on.