A rhombus is a parallelogram with four equal-length sides. A square is a rectangle with four equal-length sides.

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.

8a⁸ - 4a⁸ = 4a⁸

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 -2 as well and sum (Inside, Outside) to equal 1. For this problem, those two numbers are -1 and 2. Then, plug these into a set of binomials using the square root of the First variable (y²):

y² + y - 2
y² + (-1 + 2)y + (-1 x 2)
(y - 1)(y + 2)

Perimeter is equal to the sum of the four sides:

p = a + b + c + d
p = 1 + 7 + 9 + 4
p = 21

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).