Answer:
x =28
Step-by-step explanation:
We can use proportions to solve this problem, Put the side of the small triangle over the side of the large triangle.
x 42
------- = -----------
x+10 42 + 15
Simplify
x 42
------- = -----------
x+10 57
Using cross products
57*x = 42 (x+10)
Distrbute 42
57x = 42x+420
Subtract 42x from each side
57x -42x = 42x-42x+420
15x = 420
Divide by 15
15x/15 = 420/15
x =28
