Define watt, express kilowatt in terms …

For the numerical part,

use the formula Power = Work done / Time taken

The mass of the engine is 150 kg and it develops a power of 500 W for each kg,

So total power developed in the engine is 150*500= 75000W = 75kW

Work done = Force* Displacement

When we put this in the formula of power, we can write Displacement/Time taken=Velocity

Then, the formula of power becomes Force*Velocity

We are given the velocity of car as 20 m/s

So, Power of engine / Velocity of car = Force exerted

75000/20 = 3750 N

Watt is the SI unit of Power. 1 watt is the power of an appliance which does work at the rate of 1 Joule per second. The bigger unit of power is called as kilowatt(kW)  which means 1kW=1000W

To express kilowatt in terms of Joules we will take the commercial unit of energy kilowatt-hour (kWh) 

1 kilowatt hour=1 kilowatt for one hour

= 1000 watts per hour

But 1 watt= 1 joule/1 second 

So,  we can say 

1 kilowatt-hour = 1000 joules/seconds for 1 hour

And 1 hour = 60*60 seconds 

So,  1 kilowatt-hour = 1000 joules/seconds  * 60*60 seconds 

= 36,00,000 joules

Thus, 1kWh=3.6*10⁶ J