a+b=2 b-a=4

a+b=2 b-a=4

$$\begin{array}{ccc|c|c|}
& & & I.& II.\\
b+a & = & 2 \ & &\\
& & & + & -\\
b-a & = & 4 \ & &
\end{array}$$

I.

 $$\small{\text{
$
\begin{array}{rcl}
2b + a + (-a) & = & 2 +4 \\
2b +0 & = &6 \\
2b & = &6 \quad | \quad : 2\\
b & = & \frac{6}{2} \\
b &=& 3
\end{array}
$
}}$$

II.

$$\small{\text{
$
\begin{array}{rcl}
b - b + a - (-a) & = & 2 - 4 \\
0+2a & = &-2 \\
2a & = &-2 \quad | \quad : 2\\
a & = & \frac{-2}{2}\\
a &=& -1
\end{array}
$
}}$$

a + b = 2

   (Step 1) b = 2 - a → (Step 3) b = 2 - (-1) → b = 3

b - a = 4 

   (Step 2) 2 - a - a = 4 → 2 - 2a = 4 → -2a = 2 → a = -1 

a = -1; b = 3