Hey there! :D
The biggest thing to remember here is the similar triangles and their side lengths.
Okay, so we can separate these triangles into 3 similar triangles. Which, we only need to do that to two for a proportion, but if you were to have more variables, you could separate the triangles even more if you needed to.
We want to find two triangles that have "x" has a value of their side, so we can solve a proportion.
The bigger triangle, (the whole two triangles combine) and the smaller triangle on the left can be used to make a proportion.
On the smaller triangle:
We have 9 cm on the shorter side and x as the hypotenuse.
On the larger triangle:
We have x as the shorter side, and then 25 as the hypotenuse. (9+16)
Note: You have to flip the larger triangle upside down to make it a similar structure to the other two triangles. It is similar, you just have to flip it.
So, now we can make a proportion.
[tex] \frac{x}{9} = \frac{25}{x} [/tex]
Now, cross multiply.
9*25= 225
x*x= [tex] x^{2} [/tex]
So, find the square root.
√225= 15
x= 15.
I hope this helps!
~kaikers