If you mean endpoints of (1, 7) and (3, 3) then the midpoint is at (2, 5).
The slope is 0.
To convert seconds to milliseconds, you multiply by 1,000, since there are 1,000 milliseconds in a second. Therefore, to find the number of milliseconds in 6.7 seconds, you calculate 6.7 seconds × 1,000 ms/second, which equals 6,700 milliseconds.