Answer:
x=28
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
x 42
------ = ----------
x+10 57
Solve using cross products
57x = 42(x+10)
Distribute
57x = 42x+420
Subtract 42x from each side
57x-42x = 42x-42x+420
15x = 420
Divide by 15
15x/15 = 420/15
x=28