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 -4. 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 - 4y - 21
y² + (-7 + 3)y + (-7 x 3)
(y - 7)(y + 3)

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.

7a - 2a = 5a

Perimeter is equal to the sum of the four sides:

p = a + b + c + d
p = 8 + 5 + 4 + 5
p = 22

To solve this equation, repeatedly do the same thing to both sides of the equation until the variable is isolated on one side of the > sign and the answer on the other.

a - 4 > \( \frac{a}{5} \)
5 x (a - 4) > a
(5 x a) + (5 x -4) > a
5a - 20 > a
5a - 20 - a > 0
5a - a > 20
4a > 20
a > \( \frac{20}{4} \)
a > 5

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

a² - 4 = 0
(a - 2)(a + 2) = 0

For this expression to be true, the left side of the expression must equal zero. Therefore, either (a - 2) or (a + 2) must equal zero:

If (a - 2) = 0, a must equal 2
If (a + 2) = 0, a must equal -2

So the solution is that a = 2 or -2