Well you would divide distance by time and you should make sure that you do the trucks equation seperate from the cars equation other wise you would only get one answer and it looks like you need 2 so figure out the time an distance for each car then divide distance by time and that is your answer.
That's what I got. to rationalize the numerator you have to multiply by the conjugate, in this case (√3 + 9) / (√3 + 9), which is what it looks like you did.
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The correct answer is -x+4