Explain how to use mental math to find 530-166

Triangle xyz has vertices x(1,3), y(0,0)

Acompany that manufactures and sells guitars made changes in their product range. from the start they had 20 models. then they reduced the number of models to 15. as a result, the company enjoyed a 10% increase in turnover. - how much did the stock level change? in (%)

Prove that (x-2)is factor of p (x)=2x³-3x²-17x+30