How can we prove that sina'=nsinb using the given information and figure?

[annalian]

Homework Statement

look at the figure and prove that sina'=nsinb. We have given a, b, a', b' and n
http://s32.postimg.org/n7rnu0khx/image.png

Homework Equations

sinb'/sina'=1/n

The Attempt at a Solution

But in the equation we have b, not b'. I made the figure. How would the correct one be?

 

[RUber]

Can you prove that $\sin b = \sin b'$?

 

[annalian]

Can you prove that $\sin b = \sin b'$?

No, only if the triangle has 2 equal sides

 

[RUber]

Are the dashed lines making a and a' forming right angles with the sides of the triangle?
What other information do you have about this problem? How is the interior vector chosen/created?