Either substitution or elimination.
Use substitution to substitute an equation into another to cancel out a variable, use elimination to also add/subtract variables to cancel them out.
example:
y = x + 4
3y + x = 10
You would substitute the entire expression into the y variable for the second equation, which would look like 3(x + 4) + x = 10
Now, we have only x which is much easier to deal with than two variables.
-4x + 2y = 10
4x + 2y = 15
For elimination, we would add the two equations together to try and eliminate one variable. In this case, we can add both equations and x cancels out. Therefore, this becomes 4y = 25.