## Quadratic Parabola equations

Hi there
Would like a little help on this problem here if thats ok?
Your help is greatly appreciated!

Screen Shot 2019-01-11 at 1.57.30 PM.png Screen Shot 2019-01-11 at 1.55.27 PM.jpg

I have worked out the equation for the parabola:

x^2 = -28(y-8.5)

However, I am puzzled on how to move on from that point onwards.
Again, it's another study I am stuck on!
Unfortunately I was only given one example on this particular equation (especially a negative quadratic equation), but it doesn't suit the problem I am trying to solve on this one.
Here is that example:

Screen Shot 2019-01-11 at 2.07.51 PM.jpg
Screen Shot 2019-01-11 at 2.08.07 PM.jpgScreen Shot 2019-01-11 at 2.08.20 PM.jpg

I am unsure as to what I should do next. I am familiar with factorising and squaring but only worked on normal parabolas that already have their equations given. (The canvas is blank on this round!)
I feel that I need to substitute 6 metres somehow to give me an answer. I was trying to find a way to calculate everything that I have learnt but it doesn't seem to give me the answer I am looking for.
Trying not to rely too much on the desmos calculator but maybe I might need to for an answer.
Who knows? Would love your input on this. Cheers!