When solving an equation with two variables, replace the variables with the values given and then solve the now variable-free equation. (Remember order of operations, PEMDAS, Parentheses, Exponents, Multiplication/Division, Addition/Subtraction.)

The entire length of this line is represented by AD which is AB + BD:

AD = AB + BD

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.

3a + 3a = 6a

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

x² - 1 = 0
(x - 1)(x + 1) = 0

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

If (x - 1) = 0, x must equal 1
If (x + 1) = 0, x must equal -1

So the solution is that x = 1 or -1

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.