a)
[tex]A=8\qquad\qquad B=12.5\\\\\\d(A,B)=|12.5-8|=|4.5|=\boxed{4.5\,\text{cm}}[/tex]
b)
[tex]C=?\\\\\\d(C,A)=2\cdot d(C,B)\\\\|A-C|=2\cdot|B-C|\\\\|8-C|=2\cdot|12.5-C|\\\\
|8-C|=|25-2C|\\\\\\8-C=25-2C\qquad\vee\qquad8-C=-(25-2C)\\\\-C+2C=25-8\qquad\vee\qquad8-C=-25+2C\\\\C=17\qquad\vee\qquad8+25=2C+C\\\\C=17\qquad\vee\qquad33=3C\quad|:3\\\\\boxed{C=17\qquad\vee\qquad C=11}[/tex]
Mark 11 or 17.
