I have a question on vectors which probably doesn't belong here but I can't see an appropriate place to post it. If anyone has a better idea please tell me.

Please see image attached:

In the diagram, OABCDEFG is a cuboid in which the length of OA is 4 units, AB is 3 units and BF is 2 units. Unit vectors i, j, and k are parallel to OA, OC and OD respectively. The midpoints of AB and FG are M and N respectively.

a) Express each of the vectors ON and MG in terms of i, j, and k.

I would start this problem by finding ON→=OD→+DG→+1/2GN→

I have place the arrow to the right of the signs as I can't put them above it.

ON→=1/2i+j+k

MG=1/2AB+BF+FG

-i+1/2j+k

Am I doing the right thing anyone?

b) Find the acute angle between the direction of ON and MG, correct to the nearest 0.1°.

Will tackle this next.