Answer:
BC = 21 cm
Step-by-step explanation:
In similar triangles the corresponding sides are in same proportion.
Its given in the sum that AC = 12 cm; DF = 20 cm and EF = 35 cm.
[tex]\sf \dfrac{AB}{DE}=\dfrac{BC}{EF}=\dfrac{AC}{DF}\\\\\\\dfrac{AC}{DF}=\dfrac{BC}{EF}\\\\\dfrac{12}{20}=\dfrac{BC}{35}[/tex]
[tex]\sf \dfrac{12}{20}*35=BC\\\\\\\dfrac{12}{4}*7=BC\\\\\\3*7=BC\\\\ \boxed{BC = 21 cm}[/tex]