Answer
Please see the work below.
Work Step by Step
We know that the initial relative speed is
$v_i=80-25=55\frac{Km}{h}=15.28\frac{m}{s}$
Now, we can find the final relative speed as follows
$v_f^2=v_i^2-2ax$
We plug in the known values to obtain:
$v_f^2=(15.28)^2+(2)(-2.1)(50)=23.41$
$v_f=4.8\frac{m}{s}$
