I feel like an idiot but can anyone show me how to calculate the Inverse of Demand quantity? I not have a finance undergraduate degree and I can’t figure out what the calculation looks like.

In the Schweser books Pg 13 Book 2 they show and example of Aggregating Consumer Demand.

The demand quantity for gas is:

QDgas= 138500-12500Pgas I understand how to get to this answer but I am having difficulting getting the inverse answer of Pgas=11.08-.00008QDgas

Could someone please show me the calculation so I can figure it out.

Much appreciated. Thanks!