Given `h=(-1)/16(d-28)^2+49` where h is the height and d is the horizontal distance downrange, find d such that the height is 45m.

Set the height to 45

`(-1)/16(d-28)^2+49=45`

`(-1)/16(d-28)^2=-4` ** Subtract 49

`(d-28)^2=64` **Multiply by -64

`d-28=+-8` ** Take the square root of both sides.

Note that...

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Given `h=(-1)/16(d-28)^2+49` where h is the height and d is the horizontal distance downrange, find d such that the height is 45m.

Set the height to 45

`(-1)/16(d-28)^2+49=45`

`(-1)/16(d-28)^2=-4` ** Subtract 49

`(d-28)^2=64` **Multiply by -64

`d-28=+-8` ** Take the square root of both sides.

Note that there are two possibilities, thus the `+-`

`d=28+8` or `28-8`

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**Thus the distances where the height is 45m are d=20m and d=36m**

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