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

a = l x w
a = a x b
a = 3 x 6
a = 18

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

b² + 8b - 9 = 0
(b - 1)(b + 9) = 0

For this expression to be true, the left side of the expression must equal zero. Therefore, either (b - 1) or (b + 9) must equal zero:

If (b - 1) = 0, b must equal 1
If (b + 9) = 0, b must equal -9

So the solution is that b = 1 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.

7a + 4a = 11a

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 formula for circumference is circle diameter x π. Circle diameter is 2 x radius:

c = πd
c = π(2 * r)
c = π(2 * 4)
c = 8π