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a spacecraft moving at 0.75c fires a missle travlling at 0.30c with respect to itself at a stationary planet, what is the missles speed with respect to the planet

i think i need to use

u = v1+v2 / 1 + v1xv2 / c^2

but i am not sure if this is going to give me the speed with respect to the planet, i think i need a bit more help with how to set up the inertial frames

the frame of the spaceship is S then the missle is S' and the planet S'' ?

thanks