So we have:
$$x_1 = a, \quad y_1 = f(a), \quad x_2 = a+h, \quad y_2 = f(a+h)$$
Substituting these values into the formula, we get:
$$\text{slope} = \frac{f(a+h) - f(a)}{a+h - a}$$
Since $f(x) = 3x^2$, we can substitute $f(a)$ and $f(a+h)$:
$$\text{slope} =
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