The image is a bit hard to make out, so I'm going to make some guesses. It looks like it says

,

, and

, and you're asked to find

.
You know that

and

are similar, which means their corresponding sides are proportional in length. So there's some constant

such that



To find

, you need to know

and the constant

.
Clearly,

. So, the first equation becomes

Then in the second equation, you have