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