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Substitution is the process of replacing a variable in an expression with its actual value. If you are given an equation like 4z + 6 = x + z, told that z = 2, and asked to solve for x, what do you do? The first step is to substitute 2 for every 'z' in the problem.
This leaves us a more manageable equation, 4 * 2 + 6 = x + 2 (remember that * means multiply). After simplifying things a little we get 14 = x + 2. Subtract 2 from each side and you'll discover that x = 12.
Substitution can also involve more complicated problems, like 5x^2 + 2y -6 = y, where y = -1 and x = 4. Just substitute in the same way, placing the value of each variable in place of the letter and simplifying.
There is another form of substitution. This is where we insert a variable to stand for a particular expression. In some cases you will be dealing with incredibly complex problems, and it may be easier to substitute a variable, like y for part of the problem. Then, when you have solved the value of y, you can plug back in the original expression.