I would have thought of using a motion equation but the Energy equation you have here works nicely. At 88 km/hr, the truck has kinetic energy of Joules. We don't know the trucks mass but wait on that.

At 52 km/h, its kinetic energy is Joules. Theworkdone by the brakes is the difference between those two: Joules.

(Notice the order of those speeds. The truck slows down so the work done is negative.)

The truckdecelerates at so itsacceleration is . Force equals mass times acceleration (that formula isn't given here but I am sure you know it): .

Now use W= Fd: .

The unknown mass, m, cancels and you can solve for d.