Let's see if I can show this here...
If x+3 is a factor, then we want to test -3, because x must equal -3 for x + 3 to equal 0 and be a factor of your polynomial.
We are going to use the coefficients. The first will go straight down. We will multiply it by -3, put the answer under the next coefficent and add. Repeat.
-3| 2 5 2 5 3
| -6 3 -15 30
2 -1 5. -10 33
Because our final number is not 0, x + 3 is not a factor.
Hope this helps!
