[tex]\frac{1}{x_2}- \frac{1}{x_1}= \frac{1}{f}=> \frac{1}{x_2}=\frac{1}{f} +\frac{1}{x_1}=\frac{x_1+f}{x_1f}=>x_2=\frac{x_1f}{x_1+f} =\frac{(-20)*15}{-20+15}=60 cm=>x_2=60 cm\\\beta=\frac{x_2}{x_1}=\frac{60}{-20}=3 =>\beta=-3 \\\beta=\frac{y_2}{y_1}=>y_2=\beta x_1=(-3)*30=-90 cm=>y_2=-90 cm[/tex]
