The circumference of a circle is the distance around its perimeter and equals π (approx. 3.14159) x diameter: c = π d. The area of a circle is π x (radius)² : a = π r².

When solving an equation with two variables, replace the variables with the values given and then solve the now variable-free equation. (Remember order of operations, PEMDAS, Parentheses, Exponents, Multiplication/Division, Addition/Subtraction.)

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 solve this equation, isolate the variable for which you are solving (a) on one side of the equation and put everything else on the other side.

-6a - 5y = -2a + 9y + 9
-6a = -2a + 9y + 9 + 5y
-6a + 2a = 9y + 9 + 5y
-4a = 14y + 9
a = \( \frac{14y + 9}{-4} \)
a = \( \frac{14y}{-4} \) + \( \frac{9}{-4} \)
a = -3\(\frac{1}{2}\)y - 2\(\frac{1}{4}\)

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

AD = AB + BD