Question 74263
P = (x , y)
M = (-7 , -3)
Q = (6 , -3)


Midpoint = ( {{{ (x1 + x2)/2}}} , {{{ (y1 + y2)/2 }}} )
work the X values first
{{{ -7/1 = (x+6)/2 }}}
Cross multiply
{{{ -14 = x + 6 }}}
subtract 6
{{{ -20 = x }}}


Now work the Y values
{{{ -3/1 = (y + -3)/2 }}}
Cross multiply
{{{ -3(2) = y -3 }}}
multiply
{{{ -6 = y - 3 }}}
add 3
{{{ -3 = y }}}


Midpoint = (-20 , -3)


Your method does work as well since the Y coordinates are the same, the line is a horizontal line.  Your method will not work unless the line is horizontal or vertical.