Answer: D. 19
Let x by the length of CG. Since the two bases are parallel, we can create the following equation using proportions:
![(63)/(21)=(57)/(x)](https://img.qammunity.org/2021/formulas/mathematics/high-school/9lqlylgrqk7ayruvmghie8z1tgyac3eukg.png)
Simplify left side of equation
![3=(57)/(x)](https://img.qammunity.org/2021/formulas/mathematics/high-school/r7ycqwcld6roous2vmvowbu6my0g1ypyj3.png)
Multiply both sides by x
![3x=57](https://img.qammunity.org/2021/formulas/mathematics/high-school/7ktw1x2km3707zc2o2idj1krafqvrh7mq8.png)
Divide both sides by 3
![x=19](https://img.qammunity.org/2021/formulas/mathematics/middle-school/43yxmmv6o5v7dix0wcaifrujybk0t8usih.png)
Let me know if you need any clarifications, thanks!