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).

The surface area of a cube is (2 x length x width) + (2 x width x height) + (2 x length x height):

sa = 2lw + 2wh + 2lh
sa = (2 x 9 x 6) + (2 x 6 x 3) + (2 x 9 x 3)
sa = (108) + (36) + (54)
sa = 198

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

a = l x w
a = a x b
a = 3 x 7
a = 21

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.

3a x 2b = (3 x 2) (a x b) = 6ab

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 -9 as well and sum (Inside, Outside) to equal 0. For this problem, those two numbers are -3 and 3. Then, plug these into a set of binomials using the square root of the First variable (y²):

y² - 9
y² + (-3 + 3)y + (-3 x 3)
(y - 3)(y + 3)