Concept:
The slope of a line can be found by converting its general equation into the slope-intercept form $y = mx + c$, where $m$ represents the slope. Alternatively, for an equation in the form $Ax + By + C = 0$, the slope is $-A/B$.
Step 1: Rewrite the equation in general form.
The given equation is $\frac{x}{10} - \frac{y}{4} = 3$.
Multiply the entire equation by the LCM of $10$ and $4$ (which is $20$) to clear fractions:
\[
20 \left( \frac{x}{10} \right) - 20 \left( \frac{y}{4} \right) = 20(3)
\]
\[
2x - 5y = 60
\]
Step 2: Isolate $y$ to find the slope-intercept form.
Move $2x$ to the other side:
\[
-5y = -2x + 60
\]
Divide the entire equation by $-5$:
\[
y = \frac{-2}{-5}x + \frac{60}{-5}
\]
\[
y = \frac{2}{5}x - 12
\]
Step 3: Identify the slope.
Comparing $y = \frac{2}{5}x - 12$ with $y = mx + c$, we find:
\[
m = \frac{2}{5}
\]