3x^3+5x^2-3x-5=0
There may be other ways, but the simplest way would be to change the equation to:
3x^3+5x^2=3x+5
Now factor out the x^2 on the left and you get:
x^2(3x+5)=3x+5
then divide out the 3x+5 and you get x^2=1 so you have to check to see if either -1 or +1 is correct. Usually a polynomial will have as many solutions as the highest power. I think, but you can check your textbook, or work out a bunch that you think up to see if this is true.
use polynomial division like find the factor that can divide the equation eg X-1 and do long division on the equation using this factor so that the equation becomes a simple quadratic then factorise it you will have 3 answers of X fairly simple stuff