Subtracting 3x^4+9x^3+3x^2 from the divided and bringing down 13x.
You can use a tangent:

We have opposite = 17 and adjacent = x.

substitute:
cross multiply
multiply both sides by √3

divide both sides by 3

Use the Pythagorean theorem:

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Other method.
triangle.
The sides are in the ratio 
Therefore
