Quadratic equation (in our case ) has the following solutons:
![x[12] = (b+-sqrt( b^2-4ac ))/2\a](/cgi-bin/plot-formula.mpl?expression=x%5B12%5D+=+%28b%2B-sqrt%28+b%5E2-4ac+%29%29%2F2%5Ca&x=0003)
For these solutions to exist, the discriminant should not be a negative number.
First, we need to compute the discriminant : .
Discriminant d=465 is greater than zero. That means that there are two solutions: .
![x[1] = (-(21)+sqrt( 465 ))/2\3 = 0.0939764421413039](/cgi-bin/plot-formula.mpl?expression=x%5B1%5D+=+%28-%2821%29%2Bsqrt%28+465+%29%29%2F2%5C3+=+0.0939764421413039&x=0003)
![x[2] = (-(21)-sqrt( 465 ))/2\3 = -7.0939764421413](/cgi-bin/plot-formula.mpl?expression=x%5B2%5D+=+%28-%2821%29-sqrt%28+465+%29%29%2F2%5C3+=+-7.0939764421413&x=0003)
Quadratic expression can be factored:

Again, the answer is: 0.0939764421413039, -7.0939764421413.
Here's your graph:
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