Every straight line can wear the same uniform: $y = mx + b$ — read aloud, y equals m x plus b. $x$ is whatever you put in — gigabytes used, weeks grown, kilometres driven — and $y$ is what comes out, usually the total. Two more numbers tell you everything else. $b$ is the y-intercept, where the line starts when $x = 0$: the flat fee, the height on planting day, the fare before the wheels move. And $m$ is the slope, the charge for every single step of x.

These lines are everywhere money and growth are — phone plans, ride fares, seedlings. The uniform solves for whichever letter the question hides: $y = mx + b$ as written, $m = \dfrac{y - b}{x}$ for the rate, $b = y - mx$ for the start. One line, three questions — and this lesson asks all three.