I dont know how to do this pls help me ​

12

25-9 is 16, so one side of the bigger triangle is 16. Since the biggest triangles hypotenuse is a multiple of 5, the triangle is a 3-4-5 triangle, so the other 2 sides of the biggest triangle must be 15 and 20. Since the bottom side of the big triangle is larger, that means it equals 20. The bigger half triangles sides are 20-16-x, so now use pythagorean theorem and x is 12