To find equation of the line passing through points $ A(x_A,y_A) $ and $ B(x_B,y_B) $, we use formula:

$$ y - y_A~=~\frac{y_B - y_A}{x_B - x_A}(x-x_A) $$

In this example we have:

$$ \begin{aligned} & \left(0,~-3\right) \implies x_A = 0 ~~\text{and}~~ y_A = -3 \\[1 em] & \left(-1,~0\right) \implies x_B = -1 ~~\text{and}~~ y_B = 0 \end{aligned} $$

After substituting into the formula, we obtain:

$$ \begin{aligned} y - y_A~&=~\frac{y_B - y_A}{x_B - x_A}(x - x_A) \\[1 em] y - \left(-3\right)~&=~\frac{ 0 - \left(-3\right) }{ -1 - 0 } \left( x - 0 \right) \\[1 em]y + 3 ~&=~ -3 \left( x - 0 \right) \\[1 em]y + 3 ~&=~ -3x + 0 \\[1 em]y ~&=~ -3x-3 \end{aligned} $$

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