if you isolate y in the top equation to get y=5x+6 then you can substiture y for 5x+6 in the bottom equation because thats what y equals
as of now you have -3x+6(5x+6)=-12 but if you use the distrubite property you would get -3x+30x+36=-12
then if you subtract 36 from both sides you would get -3x+30x=-48 then combine like terms to get 27x=-48 then divide both sides by 27 to get
or x≈1.8
Answer: 
Step-by-step explanation:
Let be "x" the original volume of the solution (in milliliters) before the acid was added and "y" the volume of the solution (in milliliters) after the addition of the acid.
Set up a system of equations:

Applying the Substitution Method, you can substitute the second equation into the first equation and then solve for "x":