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:
x - (- 500) = 400
x + 500 = 400
x + 500 - 500 = 400 - 500
x + 0 = - 100
x = - 100
Check the answer by substituting the answer ( - 100) back into the equation.
- 100 - ( - 500) = 400