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:

(y - 5)(y + 4)
(y x y) + (y x 4) + (-5 x y) + (-5 x 4)
y² + 4y - 5y - 20
y² - y - 20

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.

9a³ + 7a³ = 16a³

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

a = l x w
a = a x b
a = 3 x 5
a = 15

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.

(2a)(3ab) + (6a²)(3b)
(2 x 3)(a x a x b) + (6 x 3)(a² x b)
(6)(a¹⁺¹ x b) + (18)(a²b)
6a²b + 18a²b
24a²b

The formula for area is πr²:

a = πr²
a = π(2²)
a = 4π