Question 383958
Use the midpoint formula,
{{{x[M]=(x[X]+x[Y])/2}}}
{{{2*x[M]=x[X]+x[Y]}}}
{{{x[Y]=2*x[M]-x[X]}}}
{{{x[Y]=2*13-8}}}
Similarly,
{{{y[M]=(y[X]+y[Y])/2}}}
{{{2*y[M]=y[X]+y[Y]}}}
{{{y[Y]=2*y[M]-y[X]}}}
{{{y[Y]=2*5-3}}}