moment about b is tha 510*14=7140????
The answer is actually easier found than you might be trying – lets see.
The moment of any force is the force multiplied by its perpendicular distance.
Therefore to calculate the moment of the 510N force about A we simply multiply
the 510N force by the distance from A (in other words 2m).
This will then give us the answer which is 510 x 2 in Nm units going clockwise.
We do the same for the moment around B 510 x 14 = ?Nm counter-clockwise.
Try this again and let me see what you get?