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):



Now let's connect them:



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(?,?).



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



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



Simplifying each one by multiplying by 2, we get



And then we have:



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



Edwin