The same line owns three costumes. Standard form, $Ax + By = C$ — where $A$ is the number in front of $x$, $B$ the number in front of $y$, and $C$ the plain number on the right — hides the slope in those coefficients: $m = -\dfrac{A}{B}$ — read aloud, m equals negative A over B. That minus sign is not decoration; it is the toll for moving $By$ across the equals sign. Forgetting it is this lesson's headline mistake — cheap to avoid, expensive to make.

Two more tools for the belt. The x-intercept is where a line touches down on the x-axis: set $y = 0$ and out falls $x_{\text{int}} = -\dfrac{b}{m}$, with $b$ and $m$ still meaning what they did last lesson — the starting height and the slope. And point-slope form, $y = y_1 + m(x - x_1)$ — y equals y-one, plus m times x minus x-one — builds y from any point you know: $(x_1, y_1)$ is that known point, $x$ is the spot you are asking about, and $y$ is the height there. Start where you stand, then let the slope carry you the rest of the trip.