Answer:
C
Step-by-step explanation:
Since the triangles are similar then the ratios of corresponding sides are equal, that is
[tex]\frac{BC}{EC}[/tex] = [tex]\frac{AC}{DC}[/tex], substituting values
[tex]\frac{21}{7}[/tex] = [tex]\frac{24-x}{x}[/tex] ( cross- multiply )
21x = 7(24 - x) ← distribute
21x = 168 - 7x ( add 7x to both sides )
28x = 168 ( divide both sides by 28 )
x = 6 → C
