a. \( \begin{aligned} 3 x-y & =6 \\ 3 x+y & =0\end{aligned} \)

Step 1 ：1. Add the two equations: \( (3x-y) + (3x+y) = 6 + 0 \) to eliminate variable y

Step 2 ：2. Simplify the equation after addition: \(6x = 6\)

Step 3 ：3. Solve for x: divide by 6 on both sides: \(x=1\)

Step 4 ：4. Substitute the value of x in one of the equations, for example, in the first equation: \(3(1) -y = 6 \)

Step 5 ：5. Solve for y: \(-y=3\Rightarrow y=-3\)