The
lengths of corresponding sides of similar triangles are
proportional. 
Take a look
at these triangles...
7cm 
21
cm 
We can
find the missing side, x, by
setting up a proportion. 
The first triangle's lengths are on the righthand side of the
proportion while we keep the second triangle's lengths on the left.
Also
notice that the corresponding sides are kept in the same position
on both sides of the proportion. 
When we solve
by crossmultiplying,
we find x to be 12 cm.
Remember, use
proportions to help find a
missing side when working with similar triangles.
Let's Practice
