The c/g is moment/weight and the total weight is 4*6,000Kg+2*500Kg or 25,000Kg. To get moment you need a datum, and since we're not given one we can choose one.
Let's choose the main wheels as the datum, in which case their moment is zero (4*6,000Kg*0m), and the moment of the front wheels will therefore be -10,000Kgm (2*500Kg*-10m); minus because the front wheels are 10m forward of the mains). The c/g is therefore -10,000Kgm/25,000Kg ie. -0.4m or 40cm in front of the mains.
Just to prove that the datum doesn't matter let's pick the front wheels, in which case their moment is zero (2*500Kg*0m), and the moment of the mains is 240,000Kgm (4*6000Kg*10m; positive because the mains are 10m behind the nosewheel. Now the c/g is 240,000Kgm/25,000Kg ie. 9.6m behind the nosewheel or 40cm in front of the mains.