Como fasso essa conta 2x-10=50

Can someone explain the elimination method to me for this problem?

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 (%)

Math each whole number with a rational,exponential expression