The circumference of a circle is the distance around its perimeter and equals π (approx. 3.14159) x diameter: c = π d. The area of a circle is π x (radius)² : a = π r².

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

A triangle is a three-sided polygon. It has three interior angles that add up to 180° (a + b + c = 180°). An exterior angle of a triangle is equal to the sum of the two interior angles that are opposite (d = b + c). The perimeter of a triangle is equal to the sum of the lengths of its three sides, the height of a triangle is equal to the length from the base to the opposite vertex (angle) and the area equals one-half triangle base x height: a = ½ base x height.

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

c² - 4c - 5 = 0
(c + 1)(c - 5) = 0

For this expression to be true, the left side of the expression must equal zero. Therefore, either (c + 1) or (c - 5) must equal zero:

If (c + 1) = 0, c must equal -1
If (c - 5) = 0, c must equal 5

So the solution is that c = -1 or 5

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.