To rearrange the formula \( y = mx + b \) using \( x \) as the quantity of interest, the first step you should take is to subtract \( b \) from both sides of the equation.
This gives you:
\[ y - b = mx \]
After that, you could proceed to isolate \( x \). But the correct first step from the choices provided is to subtract \( b \) from both sides.