To explain it simply, Rosie receives 20,000 in commission for the first 50,000 of the sale. However, she sold a property for 130,000 meaning she also receives 2,000 for the next 100,000. Therefore she receives $22,000 in total on her $130,000 sale.
The only thing you need to watch carefully is the divisor which in this case is x - 3. The divisor, in synthetic division, is the number that will make the binomial go to 0.
