Car A is moving at a speed of 25 m/s east relative to Car B.
Let O denote an observer. Then
[tex]v_{A/B}=v_{A/O}+v_{O/B}=v_{A/O}-v_{B/O}[/tex]
where [tex]v_{X/Y}[/tex] denotes the velocity of a body X relative to another body Y. We're told that
[tex]v_{A/O}=10\dfrac{\rm m}{\rm s}[/tex]
and
[tex]v_{B/O}=-15\dfrac{\rm m}{\rm s}[/tex]
where we take east to be to the positive direction. Then
[tex]v_{A/B}=(10-(-15))\dfrac{\rm m}{\rm s}=25\dfrac{\rm m}{\rm s}[/tex]
