The equation $3x + y = 0$ can be written in slope-intercept form ($y = mx + b$) to identify the slope ($m$).

Step 1: Solve for $y$

$3x + y = 0 \quad \Rightarrow \quad y = -3x$

Step 2: Identify the slope

From $y = -3x$, the slope ($m$) is: $\boxed{-3}$

The slope of the line is -3.

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Here are 5 related questions for you:

1. What is the y-intercept of $3x + y = 0$?
2. How do you find the slope from two points on a line?
3. What does a negative slope indicate about the direction of the line?
4. How would the slope change if the equation were $3x - y = 0$?
5. Can you convert a vertical line equation to slope-intercept form?

Tip: In slope-intercept form $y = mx + b$, the slope ($m$) represents how much $y$ changes for every unit change in $x$.