could someone help me out pls

Answer:
x = 11
Step-by-step explanation:
So you would have to set up a proportion. Since you have the measurements of one triangle, it is easy to find x.
35/42 = 15/18 = 25/3x-3
Since 35/42 and 15/18 are equal, you could choose one to find x.
Let's use 15/18.
15/18 = 25/3x-3
Cross multiply.
45x-45 = 450
45x = 495 Divide both sides by 45
x = 11