The formula for circumference is circle diameter x π:

c = πd
c = 18π

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.

To factor a quadratic expression, apply the FOIL method (First, Outside, Inside, Last) in reverse. First, find the two Last terms that will multiply to produce -8 as well and sum (Inside, Outside) to equal -2. For this problem, those two numbers are -4 and 2. Then, plug these into a set of binomials using the square root of the First variable (y²):

y² - 2y - 8
y² + (-4 + 2)y + (-4 x 2)
(y - 4)(y + 2)

Angles around a line add up to 180°. Angles around a point add up to 360°. When two lines intersect, adjacent angles are supplementary (they add up to 180°) and angles across from either other are vertical (they're equal).

According to the Pythagorean theorem, the hypotenuse squared is equal to the sum of the two perpendicular sides squared:

c² = a² + b²
c² = 2² + 6²
c² = 4 + 36
c² = 40
c = \( \sqrt{40} \)