Chapter 4 - Congruent Triangles - 4-1 Congruent Figures - Practice and Problem-Solving Exercises - Page 223: 41

$x = 5$

If $\triangle ACD$ is congruent to $\triangle ACB$, then $\angle CAD$ is congruent to $\angle CAB$. Thus, we can set them equal to one another to solve for $x$: $6x = 30$ Divide both sides by $6$ to solve for $x$: $x = 5$