Question

How do I solve this

Answer 1

If ABC is ~ to DEF then
AB is ~ to DE
35~x and
BC is ~ to EF
30~15
30/2= 15 So 35/2= 17.5

Answer 2

Both triangles are similar because of angle angle similarity.

So you can just make a proportion.

$\frac{CB}{BA} = \frac{FE}{x}$

Insert the numbers:
$\frac{30}{35} = \frac{15}{x}$

Cross multiply: 30x = 520
x = 17.5