Start from zero — the basic math you need before everything else
Most straight lines can be written in one tidy form: y = mx + b. Here m is the slope (the steepness) and b is the y-intercept, the height where the line crosses the y-axis (where x = 0). Once you know m and b, you know the whole line.
To find the y for any x, just plug the x into the formula. To read off m and b from an equation, look at the number multiplying x (that's m) and the number on its own (that's b). Making m bigger tilts the line steeper; changing b slides the whole line up or down.
The line y = mx + b describes anything that starts somewhere and changes at a steady rate. A water tank with 10 litres already in it, filling at 3 litres a minute, holds y = 3x + 10 litres after x minutes: b = 10 is the starting level, and m = 3 is the fill rate.
▶ Equation of a Line