Answer
38 m/s
Work Step by Step
Please see the attached image first.
Let's take,
Velocity of the truck relative to the ground = $V_{TG}$
Velocity of the truck relative to the police car = $V_{TP}$
Velocity of the police car relative to the ground = $V_{PG}$
(a) The vector sum represented in the image.
(b) By using the Pythagorean theorem, we can get
$V_{TP}^{2}=V_{TG}^{2}+V_{PG}^{2}$
$V_{TG}=\sqrt {V_{TP}^{2}-V_{PG}^{2}}$ ; Let's plug known values into this equation.
$V_{TG}=\sqrt {(48\space m/s)^{2}-(29\space m/s)^{2}}=38\space m/s$
