You use substitution when you can solve for one variable in terms of the others. By substituting, you remove one variable from the equation, which can then be solved.
Once you solve for one variable, you can use substitution to find the other.
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Substitution is a way to solve without graphing, and sometimes there are equations that are impossible or very difficult to graph that are easier to just substitute. Mostly though, it is a way to solve if you have no calculator or cannot use one (for a test or worksheet).
You use algebra and solve the system(s) of equations using techniques such as elimination or substitution.
There is no simple answer. Sometimes, the nature of one of the equations lends itself to the substitution method but at other times, elimination is better. If they are non-linear equations, and there is an easy substitution then that is the best approach. With linear equations, using the inverse matrix is the fastest method.
You plug in what the variable is equal to for that variable then you will be able to finish the problem
Why? - Mainly to help in solving equations.