Work. Do Not Solve

work. Do not solve.
-5x + 9y = -12
-4x - 4y = -32
A) Substitution B) Addition
Answer: B
Write the word or phrase that best completes each statement or answers the question.
-How many liters (L) of a 10% silver iodide solution must be mixed with 6 L of a 4% silver iodide solution to get a 6% solution?

A) 4.0 L
B) 3.0 L
C) 6.0 L
D) 2.0 L