First convert every thing to a consistent set of units.

Are the tons long tons or metric tons? I will assume metric tons of 1000 kg.

So the mass of the vehicle is 3000 kg.

50 km/hr = 13.89 m/s

Assume that the total force during the decelleration is constant.

Then the KE of the vehicle is 0.5 3000 13.89^2,

If we have a constant decellerating force f it does work 50 f, which

is equal to the initial KE, so:

50 f = 1500 13.89^2,

or: f=30 13.89^2= 5788.0 N

So the breaking force is this less the tractive resistance of 150x3 N or

5338 N.

RonL