From Förberedande kurs i matematik 1

The left-hand side of the equation consists of two factors, \displaystyle x and \displaystyle x+\text{3}. The only possibilities that their product is zero are if either \displaystyle x or \displaystyle x+\text{3} is zero. Hence , the equation has the solutions \displaystyle x=0\text{ } and \displaystyle x=-\text{3}.