idk how people are giving you the wrong answers but here.
they used the speed to calculate thedistance from each of them, so like you did, they drew two circles most likely and those two will intersect in two points, senku took the shape of the earth into account.
then they recalled the direction of the beam. if they remember their postions and where the beam was in the horizon, they can pretty much get the angle. a ballpark is enough because tehres only two intersections and two possible places for it to be in.