Question 145193
two ways to do this:
1) just divide x^3-5x^2-18x+72, by (x-3) and see that it goes in without a remainder. See this URL  http://www.calc101.com/webMathematica/long-divide.jsp

2) set x=3 and solve the equation. If the answer is 0, then 3 is a (x-3) is a factor
(27- 5*9 - 18*3 + 72 = ???) if the answer is 0, the 3 is a root and (x-3) is a factor