Since the triangles are similar, then their corresponding sides satisfy the next proportion:
[tex]r=\frac{YZ}{BC}=\frac{YX}{BA}=\frac{XZ}{AC}[/tex]Substituting with data, we get:
[tex]r=\frac{21}{12}=1.75[/tex][tex]\begin{gathered} \frac{21}{12}=\frac{z}{7} \\ \frac{21}{12}\cdot7=z \\ 12.25\text{ m = z} \end{gathered}[/tex][tex]\begin{gathered} \frac{21}{12}=\frac{17.5}{b} \\ 21\cdot b=17.5\cdot12 \\ b=\frac{17.5\cdot12}{21} \\ b=10\text{ m} \end{gathered}[/tex]