Answer:
x=75
Explanation:
We can use proportions to solve this problem. Put the side of the small triangle over the side of the large triangle.
42 50
------ = ----------
42+63 50+x
42 50
------ = ----------
105 50+x
Using cross products
42(50+x) = 50*(105)
2100+42x = 5250
Subtract 2100 from each side
2100-2100+42x = 5250-2100
42x =3150
Divide by 42
42x/42 = 3150/42
x = 75