For two points $\mathrm{A}(a, -2, 6)$ and $\mathrm{B}(9, 2, b)$ in coordinate space, the midpoint of segment AB has coordinates $(4, 0, 7)$. What is the value of $a + b$? [2 points] (1) 1 (2) 3 (3) 5 (4) 7 (5) 9
For two points $\mathrm{A}(a, -2, 6)$ and $\mathrm{B}(9, 2, b)$ in coordinate space, the midpoint of segment AB has coordinates $(4, 0, 7)$. What is the value of $a + b$? [2 points]\\
(1) 1\\
(2) 3\\
(3) 5\\
(4) 7\\
(5) 9