hello i have 2 motors this running at 7,2 volt battery , 3 ampere for one.
the battery is dead days later and don't have another. but i have a lot of 12v battery can use them and do not light fires? 
the resistance required is
vA=7,22,5=18w for running one (no load)
vA=7,24,5=32wfor start one ( its motor brushes)
quindi R = 4,8 / 4= 1,2 Ohm
Where can I find two resistors 30w/color]
is for rc boat 
Classic beginners mistake.
Do not use resistors for cutting down voltage.
Use a regulator. In your case if you use a switching regulator you will avoid burning off the excess power. Look for DC to DC converters, there are lots about.
Unless you have good motor control your other problem is that the starting current of the motors is going to be several times the rated running current, so probably in the 10s of amps. This means your convertor needs to be over-sized for the normal running current. Similarly if you tried using a simple resistor dropper it would have too great an effect upon start-up.
You'd be better off buying some new 7.2volt batteries than trying to cobble up an alternative system. Apart from that, how do you fit a 12 volt battery of sufficient AH capacity into a boat designed for a neat Nicad or whatever battery pack.
Alternatively get a 7.2volt charger that operates from a 12 volt car battery
I most worried by the batteries dying in a matter of days - what happened? How will you avoid a repeat of this?