There are three main methods to solve a system of equations:
- Elimination method
- Graphing method
- Substitution method
The first one, Elimination, consists in add or subtracting the equations together to find the value of one of the variables. This is what option A says, and for this reason, this is not the answer.
Option C is also a step to the Elimination method, which needed to be done before adding the two-equation with the intent to isolate X. For this reason, this is not the answer also.
Option D would be equivalent to option C, and also a possible first step for the Elimination method.
Substituting consists of isolating one of the variables in any of the equations, by your choice, then substituting it in the other equation. This first process can also be called "Solve the equation for one of the variables", in a more generic way it can be said "Solve the equation", and for this reason, the correct answer is :
B: Solve the first equation
It is good the remember that it could be any of them, not necessarily the first equation.