Question 107347
Since the y coordinate is the same for both (y=0), the y coordinate of the midpoint will also be 0.
The x coordinate is the average of the two x coordinates. 
{{{x[m]=(x[1]+x[2])/2}}}
{{{x[m]=(11.5+(-11.5))/2}}}
{{{x[m]=0}}}
The midpoint is (0,0).
{{{drawing( 300, 300, -15, 15, -2, 2,grid( 1 ),circle( -11.5, 0, .4 ),circle( 11.5, 0, .4 ),circle( 0, 0, .4 ),
green(line( -11.5, 0, 11.5, 0)))}}}