#### Answer

$a=6; b=7.5$

#### Work Step by Step

1. Because we have isosceles triangles, here, so it is easy to see that a=6
2. Make following three equivalent ratios
$\frac{6}{12} =\frac{1}{2} =\frac{b}{15}$
3.Then find the side b
$\frac{1}{2} =\frac{b}{15}$
$2b=15$
$b=7.5$