Finding x on a polynomial curve.

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Soldato
Joined
14 Apr 2008
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2,654
Location
Exeter
Hi all, I'm struggling to use a polynomial curve equation to find x for a given y value.

My curve equation is: y = -0.0009x^2 + 1.9113x + 4.2096

As an example, my first y = 765, but I can't for the life of me work out the x value.

I know the quadratic equation: x=(-b+√b2-4ac)/2a, but I don't know where the y value comes into it, as this equation only solves for where y = 0.

Any help would be greatly appreciated! :)
 
Well put in the value of y with your given value then unpick the equation to show x = instead of y =

Its been 4 years since iv done a-level maths but it looks fairly simple to do :)
 
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