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