The first step to solve a quadratic equation that's set to zero is to factor the quadratic equation:

a² - 81 = 0
(a - 9)(a + 9) = 0

For this expression to be true, the left side of the expression must equal zero. Therefore, either (a - 9) or (a + 9) must equal zero:

If (a - 9) = 0, a must equal 9
If (a + 9) = 0, a must equal -9

So the solution is that a = 9 or -9

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.

9a⁶ - 8a⁶ = 1a⁶

To multiply binomials, use the FOIL method. FOIL stands for First, Outside, Inside, Last and refers to the position of each term in the parentheses.

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) + (3a²)(4b)
(9 x 9)(a x a x b) + (3 x 4)(a² x b)
(81)(a¹⁺¹ x b) + (12)(a²b)
81a²b + 12a²b
93a²b

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).