Hello,

First of all, find the equation of the line between A(1,2) and B(b,6) (by solving y=mx+n)

Then, let H be the intersection of the two lines.

H is the middle of [AB]. This may give you a relation between a and b.

For the second one, you can take a random point C on y=ax+14 (in function of a) and say that CA=CB