A line on the coordinate grid can be defined by a slope-intercept equation: y = mx + b. For a given value of x, the value of y can be determined given the slope (m) and y-intercept (b) of the line. The slope of a line is change in y over change in x, \({\Delta y \over \Delta x}\), and the y-intercept is the y-coordinate where the line crosses the vertical y-axis.

A circle is a figure in which each point around its perimeter is an equal distance from the center. The radius of a circle is the distance between the center and any point along its perimeter. A chord is a line segment that connects any two points along its perimeter. The diameter of a circle is the length of a chord that passes through the center of the circle and equals twice the circle's radius (2r).

The entire length of this line is represented by AD which is AB + BD:

AD = AB + BD

The first step to solve a quadratic equation that's set to zero is to factor the quadratic equation:

x² - 2x - 8 = 0
(x + 2)(x - 4) = 0

For this expression to be true, the left side of the expression must equal zero. Therefore, either (x + 2) or (x - 4) must equal zero:

If (x + 2) = 0, x must equal -2
If (x - 4) = 0, x must equal 4

So the solution is that x = -2 or 4

The coordinate grid is composed of a horizontal x-axis and a vertical y-axis. The center of the grid, where the x-axis and y-axis meet, is called the origin.