In this case, the answer is very simple.
Defines substitution.
The answer is:
The substitution method for solving systems of linear equations, consists of solving or isolating one of the unknowns (for example, x) and substitute its expression in the other equation.
In this way, we will obtain a first degree equation with the other unknown (y), and once solved ''y'', we calculate the value of ''x'' substituting the value of ''y'', that we already know.