For any vectors question it will always help to draw out how it looks like to give yourself a better idea of what is happening. From the diagram, you can see that vector OM lies on AB and CD since it is the intersection. This hints us to use simultaneous equation to solve this question. For me, i opted for the use of vector line equations. You first find the line equation for both AB and CD then just substitute into each other since OM is a common point. There are other methods to go around this but the main concept is that OM lies on both AB and CD. (Another way can be the use of Ratio Theorem if I’m not wrong. Haven’t tried it.)