First we need to remember that:
In general, a midsegment of a triangle is a line segment which joins the midpoints of two sides of the triangle. It is parallel to the third side and has a length equal to half the length of the third side.
This means that RT has to be half of AB, then:
[tex]RT=\frac{1}{2}AB[/tex]
Solving for AB we have:
[tex]AB=2RT[/tex]
