a)

$\{\begin{array}{l}2d-3=3f-2\\ d\left(2f+5\right)-2f\left(d+3\right)=2d+1\end{array}$  

$\{\begin{array}{l}2d-3f=1\\ 2df+5d-2df-6f=2d+1\end{array}$   

$\{\begin{array}{l}2d-3f=1\mid \cdot \left(-2\right)\\ 3d-6f=1\end{array}$   

$\{\begin{array}{l}-4d+6f=-2\\ 3d-6f=1\end{array}$  

Dodajmy stronami oba równania:

$-4d+6f+3d-6f=-2+1$  

$-d=-1$  

$d=1$  

Podstawmy d do jednego z równań:

$2d-3f=1$  

$2\cdot 1-3f=1$  

$2-3f=1$  

$-3f=-\frac{1}{\mid }:\left(-3\right)$  

$f=\frac{1}{3}$  

$\{\begin{array}{l}d=1\\ f=\frac{1}{3}\end{array}$  

b)          

$\{\begin{array}{l}\frac{i-1}{s-6}=\frac{i+15}{s+2}\\ \frac{i}{s-1}=\frac{i-3}{s-4}\end{array}$  

$\{\begin{array}{l}\left(i-1\right)\left(s+2\right)=\left(i+15\right)\left(s-6\right)\\ i\left(s-4\right)=\left(i-3\right)\left(s-1\right)\end{array}$  

$\{\begin{array}{l}is+2i-s-2=is-6i+15s-90\\ is-4i=is-i-3s+3\end{array}$