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 area of a rectangle is equal to its length x width:

a = l x w
a = a x b
a = 2 x 3
a = 6

A quadrilateral is a shape with four sides. The perimeter of a quadrilateral is the sum of the lengths of its four sides.

The area of a parallelogram is equal to its length x width:

a = l x w
a = a x b
a = 1 x 5
a = 5

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.

6a⁷ + 7a⁷ = 13a⁷