A ship currently at positionais sailing on a straight course with velocityu. Simultaneously a submarine at positionbmanoeuvres to intercept the ship. If the ship maintains its straight course at all times, show that the intercept course is orthogonal to the direction (a-b) and that in order to the ship the submarine must have minimum speed

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I'm not too worried about this as I have the answers right in front of me; however the bit I am having trouble with is this part:

intercept course

I don't understand how and any help would be appreciated.