Question 1022841
Not sure how you mean, "width at ground level".  Your arch is represented as a parabola, with a vertex MINIMUM.  Your arch opens upward.  How are you expecting your width?


Your arch as described goes below the ground.
The graph of your parabola as defined is this:
{{{graph(300,300,-5,5,-9,1,0.7(x-3.5)(x+3.5))}}}




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Maybe "width" is the wrong word, and you meant "length" instead.  This may be the distance between the two x-axis intercepts, being 7 feet.