I need help on figuring a question out, so I wrote a similar question, please walk me through it so I can solve the actual one

Question

Rena donated a rectangular tent for the Medical Frontliners, which has a volume of (x³ + 11x² + 39x + 45) cubic meters. What is the length of the rectangular tent if its width and height are both (x+3) meters.

Answer 1

use long or synthetic division, divide x+3 into the cubic polynomial to get

x^2+8x+15 = (x+3)(x+5), divide again by x+3 to get

x+5 is the length