The captain of a the spaceship Red Dwarf is travelling directly towards Earth at a speed of 0.72c relative to the planet. At the same time the spaceship Soylent Green is flying directly away from Earth, directly towards the Red Dwarf, with a speed of 0.87c relative to the planet. According to the captain of the Red Dwarf, how fast is the Soylent Green travelling? Give your answer as a multiple of 'c', the speed of light in vacuum.
what I did is
ux`=ux-v/(1-vux/c^2)
and there fore
= -0.72c-0.87c / (1-(0.87*-0.72)/3.0e^8^2))
and got -1.5c
which is not right