Question 101984
{{{D=sqrt((x[2]-x[1])^2 + (y[2]-y[1]) ^2))}}}
{{{D^2=(x[2]-x[1])^2 + (y[2]-y[1]) ^2)}}}     Square both sides to remove square root.
{{{D^2-(y[2]-y[1]) ^2=(x[2]-x[1])^2 + (y[2]-y[1]) ^2-(y[2]-y[1])^2}}} Use the additive inverse of {{{(y[2]-y[1])^2}}}.
{{{D^2-(y[2]-y[1]) ^2=(x[2]-x[1])^2}}}     Simplify.
+/-{{{sqrt(D^2-(y[2]-y[1]) ^2)=(x[2]-x[1])}}}     Take the square root of both sides, could be + or -. 
{{{x[1]}}}+/-{{{sqrt(D^2-(y[2]-y[1]) ^2)= x[2]}}}     Use the additive inverse of {{{x[1]}}}} 
{{{x[2]=x[1]}}}+/-{{{sqrt(D^2-(y[2]-y[1])^2)}}}     Final answer.