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

a² - 4a - 21 = 0
(a + 3)(a - 7) = 0

For this expression to be true, the left side of the expression must equal zero. Therefore, either (a + 3) or (a - 7) must equal zero:

If (a + 3) = 0, a must equal -3
If (a - 7) = 0, a must equal 7

So the solution is that a = -3 or 7

A cube is a rectangular solid box with a height (h), length (l), and width (w). The volume is h x l x w and the surface area is 2lw x 2wh + 2lh.

A circle is a figure in which each point around its perimeter is an equal distance from the center. The radius of a circle is the distance between the center and any point along its perimeter. A chord is a line segment that connects any two points along its perimeter. The diameter of a circle is the length of a chord that passes through the center of the circle and equals twice the circle's radius (2r).

The Pythagorean theorem defines the relationship between the side lengths of a right triangle. The length of the hypotenuse squared (c²) is equal to the sum of the two perpendicular sides squared (a² + b²): c² = a² + b² or, solved for c, \(c = \sqrt{a + b}\)

A cylinder is a solid figure with straight parallel sides and a circular or oval cross section with a radius (r) and a height (h). The volume of a cylinder is π r²h and the surface area is 2(π r²) + 2π rh.