Set up a proportion. The corresponding sides are proportional.

1 3

-- = -- {1 in small triangle is to 3 in big triangle, as 3 in small triangle is to x in big triangle}

3 x

x =

1 3

-- = -- {1 in small triangle is to 3 in big triangle, as 3 in small triangle is to x in big triangle}

3 x

x =

**9**{cross-multiplied}**K.)***- Algebra House*