Can some please tell me what this is?

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 (%)

Which equation results from adding the equations in this system? x+6y=9 and -x+2y=-15

What is the total area of the figure?