From the picture you can see that the weights are as follows:

1750N

4050N

4080N

Now you need to find there moment. The moment is found by the distance the weight is acting from the datum multiplied by the weight.

1750N x 1.5 = 2625Nm

4050N x 0.0 = 0 Nm

4080N x 0.0 = 0Nm

To find the arm then it is the Sum of the moments / Sum of weight

2625/9880 = 0.26569m

Now convert it to cm (x 100)

= 26.57cm fwd of the datum