Answer:
[tex]x=10.5[/tex]
Step-by-step explanation:
Use Thales' Theorem which says: If two lines, not necessarily parallel, are cut by a system of parallel lines, then the segments that result on one of the two lines are proportional to the corresponding segments obtained on the other.
Take a look at the picture I attached you, from it, It is true that:
[tex]\frac{AB}{A'B'} =\frac{BC}{B'C'}[/tex]
Hence:
[tex]\frac{12}{6} =\frac{21}{x}[/tex]
Solving for x:
[tex]x=\frac{21}{2} =10.5[/tex]
