Answer:
 u = - 40 ft / s
Explanation:
The Galilean relation for the relative velocity is
     v ’= v + u
where u is the speed between the two reference frames, v is the speed of the fixed system and v 'the speed of the mobile system.
In this case the truck has a speed with respect to the ground (fixed system) 0 m / s (it is stopped), the car has a speed with respect to the ground of v = 40 ft / s,
    u = v'- v
    Â
    u = 0 - 40
    u = - 40 ft / s
the speed perceived by the car if the system is fixed on it is -40 ft / s