Two space ships (F and M) start at point x=x'=0 and t=t'=0. F stays put while M takes off in the x direction at 0.6c. After 350 min on F's clock, M turns around and heads back (assuming instantly). 350 more min on F's clock and M is back at start. T = 700 min. calculate time t' that has elapsed on M's clock for the first half of the trip. Now I would assume this is just where you use the Lorenz factor isnt it? So that M's time should measure shorter than 350 min for the first half.. but in the question it has the hint: Calculate M's time coordinate t', for the point tr=350 min, xr=v*tr shouldnt it just be... Gamma = 1.25, so t' = 280??