A parallelogram is a quadrilateral with two sets of parallel sides. Opposite sides (a = c, b = d) and angles (red = red, blue = blue) are equal. The area of a parallelogram is base x height and the perimeter is the sum of the lengths of all sides (a + b + c + d).

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.

6a - 3a = 3a

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 -16 as well and sum (Inside, Outside) to equal 0. For this problem, those two numbers are -4 and 4. Then, plug these into a set of binomials using the square root of the First variable (y²):

y² - 16
y² + (-4 + 4)y + (-4 x 4)
(y - 4)(y + 4)

To multiply monomials, multiply the coefficients (the numbers that come before the variables) of each term, add the exponents of like variables, and multiply the different variables together.

(5a)(5ab) - (8a²)(6b)
(5 x 5)(a x a x b) - (8 x 6)(a² x b)
(25)(a¹⁺¹ x b) - (48)(a²b)
25a²b - 48a²b
-23a²b

A triangle is a three-sided polygon. It has three interior angles that add up to 180° (a + b + c = 180°). An exterior angle of a triangle is equal to the sum of the two interior angles that are opposite (d = b + c). The perimeter of a triangle is equal to the sum of the lengths of its three sides, the height of a triangle is equal to the length from the base to the opposite vertex (angle) and the area equals one-half triangle base x height: a = ½ base x height.