Question 455941
<pre>
Let's plot the given two points.  Let's call the left 
end point L(2,9), and the midpoint M(11/2,17/2):

{{{drawing(400,400,-3,11,-3,11,

graph(400,400,-3,11,-3,11),
circle(2,9,.1), circle(5.5,8.5,.1), circle(9.8,.1),
locate(2,9.8,L(2,9)), locate(5.5,10,M(11/2,17/2))
 


)}}}

Now let's connect them:

{{{drawing(400,400,-3,11,-3,11,

graph(400,400,-3,11,-3,11),
circle(2,9,.1), circle(5.5,8.5,.1), circle(9.8,.1),
locate(2,9.8,L(2,9)), locate(5.5,10,M(11/2,17/2)), 
line(2,9,5.5,8.5) 

)}}}

Now let's extend it so that M looks like it is
the midpoint, and we'll label the unknown right 
endpoint and label it with unknown coordinates R(?,?).

{{{drawing(400,400,-3,11,-3,11,

graph(400,400,-3,11,-3,11), circle(9,8,.1),
circle(2,9,.1), circle(5.5,8.5,.1), circle(9.8,.1),
locate(2,9.8,L(2,9)), locate(5.5,10,M(11/2,17/2)), 

locate(9,8.8,R("?","?")), line(2,9,9,8) 

)}}}

Let's let the coordinates of the unknown right endpoint
R(?,?) be (a,b) so we have the point R(a,b)

{{{drawing(400,400,-3,11,-3,11, circle(9,8,.1),


graph(400,400,-3,11,-3,11),
circle(2,9,.1), circle(5.5,8.5,.1), circle(9.8,.1),
locate(2,9.8,L(2,9)), locate(5.5,10,M(11/2,17/2)), 

locate(9,8.8,R(a,b)), line(2,9,9,8) 

)}}}

Now we use the midpoint formula and get the two equations:

{{{system((2+a)/2=11/2,(9+b)/2=17/2)}}}

Simplifying each one by multiplying by 2, we get

{{{system(2+a=11,9+b=17)}}}

And then we have:

{{{system(a=9,b=8)}}}

So the right endpoint is R(a,b) = R(9,8)

{{{drawing(400,400,-3,11,-3,11,

graph(400,400,-3,11,-3,11), circle(9,8,.1),

circle(2,9,.1), circle(5.5,8.5,.1), circle(9.8,.1),
locate(2,9.8,L(2,9)), locate(5.5,10,M(11/2,17/2)), 

locate(9,8.8,R(9,8)), line(2,9,9,8) 

)}}}

Edwin</pre>