Use the midpoint formula
For (x₁, y₁) and (x₂, y₂), the midpoint is ((x₁ + x₂)/2, (y₁ + y₂)/2). It sits halfway along the straight segment joining the points.
A worked example
Between (2, 8) and (10, 4), average the x-values: (2 + 10)/2 = 6. Average the y-values: (8 + 4)/2 = 6. The midpoint is (6, 6).
Keep negative signs
Between x-values −6 and 2, the average is (−6 + 2)/2 = −2. Do not ignore the minus sign or average only the absolute values.