Answer:
15.75
Step-by-step explanation:
Triangle ABC is similar to triangle XBY because all their angles are equal. You can see this using the parallel lines. Since they are similar, the ratios of their corresponding sides are equal.
10.5/10.5+4=x/6+x
10.5/14.5=x/6+x
21/29=x/6+x
Cross-multiplying yields:
21(6+x)=29x
126+21x=29x
8x=126
x=126/8
x=15.75
