Excellent! $$v_{\text{AB}} = -v_{\text{BA}}$$. As $$v_{\text{CE}} = 30$$, $$v_{\text{EC}} = -30$$
If you have more than two objects, there is an equation that relates all the relative velocities: $$v_{\text{AB}} = v_{\text{AC}} + v_{\text{CB}}$$.
If Car X is moving at $$40 \, \mathrm{km/h}$$ wrt the Earth and Car Y is moving at $$30 \, \mathrm{km/h}$$ wrt the Earth, what is the velocity of Car X wrt Car Y. That is, what is $$v_{\text{XY}}$$ in $$\mathrm{km/h}$$?
• $$40 + 30$$
• $$(-40) + (-30)$$
• $$40 + (-30)$$
• $$(-40) + 30$$