18x^3+51x^2+26x+15

1 answer

Decartes' Rule of signs says there are no positive roots.
The Rational Roots Theorem says that the rational roots must be of the form
±(1,3,5)/(1,2,3,6)
You can quickly see that there are no rational roots, so a numeric method is your only choice left.

What was your question, anyway?