The choice of datum does not affect the result of the calculation. Below we will calculate for both, if datum was at main gear and nose gear, to show you that the result is not affected. However, it is up to you to choose which datum you want since the question does not specify a datum position.

CG = total moment / total mass

FOR DATUM AT NOSE:

Total mass = 435 kg + 2385 kg + 2415 kg = 5235 kg

Moment = mass x arm
435 kg x 0 cm = 0 kgcm (datum at nose, therefore, arm = 0 cm)
2385 kg x 583 cm = 1 390 455 kgcm
2415 kg x 583 cm = 1 407 945 kgcm

Total moment = 0 + 1 390 455 + 1 407 945 = 2 798 400 kgcm

CG = 2 798 400 kgcm / 5235 kg = 535 cm

=> The CG is located 535 cm aft of the datum (nose) – This option is not available. Therefore, we must relate it to the main gear. Main gear is 583 cm aft of the datum and the CG is 535 cm – 583 cm = -48 cm (forward) of the main gear.

FOR DATUM AT MAIN GEAR:

Total mass = 5235 kg

Moment = mass x arm
435 kg x - 583 cm = -253 605 kgcm *
2385 kg x 0 cm = 0 kgcm (datum at main gear, therefore, arm = 0 cm)
2415 kg x 0 cm = 0 kgcm (datum at main gear, therefore, arm = 0 cm)

* “-“ because nose gear is located forward of the datum.

CG = -253605 kgcm / 5235 kg = -48 cm

=> The CG is located 48 cm forward of the main gear