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

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 21 as well and sum (Inside, Outside) to equal -10. For this problem, those two numbers are -7 and -3. Then, plug these into a set of binomials using the square root of the First variable (y²):

y² - 10y + 21
y² + (-7 - 3)y + (-7 x -3)
(y - 7)(y - 3)

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.

(9a)(9ab) - (7a²)(2b)
(9 x 9)(a x a x b) - (7 x 2)(a² x b)
(81)(a¹⁺¹ x b) - (14)(a²b)
81a²b - 14a²b
67a²b

A rectangle is a parallelogram containing four right angles. Opposite sides (a = c, b = d) are equal and the perimeter is the sum of the lengths of all sides (a + b + c + d) or, comonly, 2 x length x width. The area of a rectangle is length x width. A square is a rectangle with four equal length sides. The perimeter of a square is 4 x length of one side (4s) and the area is the length of one side squared (s²).

The volume of a cylinder is πr²h:

v = πr²h
v = π(2² x 4)
v = 16π