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 2000 + (- 6000) = - 4000.

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. 2000 + x = - 4000).

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:
- 5000 + x = 4000
- 5000 + x + 5000 = 4000 + 5000
0 + x = 9000
x = 9000
Check the answer by substituting the answer (9000) back into the equation.
- 5000 + 9000 = 4000