We first have to find 2 pairs of sides that proportional.
35/20=42/24=7/4
and 21/b should be 7/4 as well because if we want the triangles to be similar the sides should be in proportion.
21/b=7/4
84=7b
b=12
Done!
Hope this helped, any questions about this just ask in the comments and I will answer.
