To rearrange the formula \( y = mx + b \) to solve for \( x \) using \( x \) as the quantity of interest, the first step you would take is:
Subtract \( b \) from both sides of the equation.
This step will allow you to isolate the term containing \( x \) on one side. After this, the equation would look like \( y - b = mx \), which you can then use to solve for \( x \).