Triangle Proportionality Theorem
If a line parallel to one side of a triangle intersects the other two sides of the triangle, then the line divides these two sides proportionally.
If 
Example :
Find the value of x.
The lines 
are parallel.
Therefore, by the Triangle Proportionality Theorem,
Substitute the values and solve for x.
Cross multiply.
6x = 18
Divide both sides by 6.
The value of x is 3.
