The two triangles are similar Triangles
similar triangles because line having length "y" is parallel to line having length "18.7 cm" and they have a common angle at top vertex.
The line with length 3.4cm and 4.1cm is a straight line which acts as a transversal cutting those parallel lines which means those parallel lines make the same angle with that line. The same goes to the other line and other two angles are also equal.
Two triangles having same angles but different side lengths are similar . so we can write
3.4/(3.4+4.1) = x/(x+6.7) = y/18.7 = factor
i added to get the total length of the sides of big triangles
for details on http://staff.argyll.epsb.ca/jreed/math9/strand3/triangle_congruent.htm