Find a linear demand equation that shows the number of swords q that can be sold per week, if the price per sword is p dollars.

Find a linear demand equation that shows the number of swords q that can be sold per week, if the price per sword is p dollars. You experimented with the prices, and found that if you charge $500 for a sword, then you can sell 75 swords per week; and that if you charge $700, then you can sell 65 per week.

I know how to solve it but I'm looking for advice on how to create a linear equation for this? Is there a step by step process or formula or is it just guess and check?