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).

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).

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 36 as well and sum (Inside, Outside) to equal 13. For this problem, those two numbers are 4 and 9. Then, plug these into a set of binomials using the square root of the First variable (y²):

y² + 13y + 36
y² + (4 + 9)y + (4 x 9)
(y + 4)(y + 9)

Perimeter is equal to the sum of the four sides:

p = a + b + c + d
p = 3 + 4 + 7 + 6
p = 20

To combine like terms, add or subtract the coefficients (the numbers that come before the variables) of terms that have the same variable raised to the same exponent.

7a + 8a = 15a