You need to be more specific.
example: A train A moving from north  to  south with a velocity of 60km/hr and trainB moving from south to north with a velocity of 70km/hr. what is velocity of B relative to A?

consider south to north a positive direction.
velocity of train B(VB)=+70km/hr
velocity of train A(VA)=-60km/hr

So B relative to A= VB-VA = 70-(-60)= 130km/hr.

Hope i could help you.

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 V_{ab} =  V_{a} - V_{b}