Question 384081
M (9, 8) is the midpoint of RS. The coordinates of S are (10, 10). What are the coordinates of R?

Use the midpoint formula,
{{{x[M]=(x[R]+x[S])/2}}}
{{{2*x[M]=x[R]+x[S]}}}
{{{x[R]=2*x[M]-x[S]}}}
{{{x[R]=2*9-10}}}
Similarly,
{{{y[M]=(y[R]+y[S])/2}}}
{{{2*y[M]=y[R]+y[S]}}}
{{{y[R]=2*y[M]-y[S]}}}
{{{y[R]=2*8-10}}}