book Maths Puzzle Numerical Ability

If a parabola goes through points (-1,3), (2,7), (4,-3), what is its vertex.

I am lost on this one. I thought I had it but I can not find it.
Thanks for your help
-Ryan

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book Other Question

Can you help me factor the polynomial 8a^3b+12a^2b^2-4a^2b Joe digs a ditch in 3.2 hours.
Joe working with George can did the same length ditch in 2.7 hours.
How long would it take George to dig the ditch by himself.