An equation is a mathematical statement that has an expression on the left side of the equals sign (=) with the same value as the expression on the right side. An example of an equation is 20 + (- 60) = - 40.

One of the terms in an equation may not be know and needs to be determined. Often this unknown term is represented by a letter such as x. (e.g. 20 + x = - 40).

The solution of an equation is finding the value of the unknown x. To find the value of x we can use the subtractive equation property which says: "The two sides of an equation remain equal if the same number is subtracted from each side." We may also use the additive equation property which says: "The two sides of an equation remain equal if the same number is added to each side."

Example:
- 50 + x = 40
- 50 + x + 50 = 40 + 50
0 + x = 90
x = 90
Check the answer by substituting the answer (90) back into the equation.
- 50 + 90 = 40