What is the first step in solving a system of equations using substitution?

The first step in solving a system of equations using substitution is to isolate one variable in one of the equations. This process involves rearranging either of the given equations to solve for one variable in terms of the other variable. Once a variable has been isolated, it can be substituted into the second equation, allowing you to solve for the remaining variable. This method is particularly useful as it simplifies the problem and allows for a more straightforward resolution of the system.

For instance, if you have a system of equations, isolating 'y' in terms of 'x' means expressing 'y' as a function of 'x', which can then be plugged into the other equation to find 'x' easily.

The other methods listed do not align with the substitution method. Multiplying the equations together or adding them does not directly lead to a substitution approach, and graphing the equations, while a valid method of solving a system, is not part of the substitution process. The essence of substitution lies in the ability to take one variable and express it in a way that allows for easy substitution into another equation.