Помогите с 12......................................................

Решите задачу:

$\frac{a^2-4b^2}{2ab}:(\frac{1}{2b}-\frac{1}{a})=\frac{(a-2b)(a+2b)}{2ab}:\frac{a-2b}{2ba}=\frac{(a-2b)(a+2b)}{2ab}\cdot \frac{2ab}{a-2b}=\\\\=a+2b=5 \frac{11}{7}+2\cdot 7 \frac{3}{17}=\frac{46}{7}+\frac{2\cdot 122}{17}=\frac{46\cdot 17+7\cdot 244}{7\cdot 17}=\\\\=\frac{2490}{119}=20 \frac{110}{119}$