Question

-x-9y=12
-4x+9y=3

Answer 1

You got a "system" of equations, try to rewrite one of the equations - isolating for example x.
Then you plug in this into your other equation, do you follow?

Answer 2

This means you rewrite on of the 2 equations so that you have: x=(whatever)
and then you plug in (whatever) into where x exists on the second equation.
You "replace" the x with (whatever). Do NOT replace the value infront of x though.
Good luck! :)