If RX=12x + 9 and RT=36, what is the value of x?

What is the missing step in this proof ?

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 it's appreciated. (picture provided)