An equation is a mathematical statement such that the expression on the left side of the equals sign (=) has the same value as the expression on the right side. An example of an equation is 200 + (- 600) = - 400.

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. 200 + x = - 400).

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:
- 500 + x = 400
- 500 + x + 500 = 400 + 500
0 + x = 900
x = 900
Check the answer by substituting the answer (900) back into the equation.
- 500 + 900 = 400