b = 10.03, z= 12.25
1) Similar triangles have proportional sides and congruent angles. Therefore we can write out the following ratios:
![\begin{gathered} (BC)/(YZ)=(CA)/(XZ) \\ (12)/(21)=(b)/(17.5) \\ 21b=12\cdot17.5 \\ (21b)/(21)=(12)/(21)\cdot(17.5)/(21) \\ b=10.03 \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/4j0yvu4bzh34zv042tsk9xzffqu67d3ud1.png)
2) And for the other missing side, we can write out this proportion:
![\begin{gathered} (BC)/(AB)=(ZY)/(XY) \\ (12)/(7)=(21)/(z) \\ 12z=7\cdot21 \\ (12z)/(12)=(7\cdot21)/(12) \\ z=12.25 \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/jd0o4fs2kjya2j59cru64ni4e8hrrafvb3.png)
3) Hence, the missing sides are b = 10.03 (rounded off to the nearest hundredth) and z= 12.25