a. ${R_{td}} = \frac{{{R_1}.{R_2}}}{{{R_1} + {R_2}}} = \frac{{15.30}}{{15 + 30}} = 10\Omega $
b. $I = \frac{U}{{{R_{td}}}} = \frac{{15}}{{10}} = 1,5A$
c. $\begin{array}{l}
{R_{td2}} = \frac{U}{{{I_2}}} = \frac{{15}}{{0,75}} = 20\Omega \\
{R_{td2}} = {R_{td}} + {R_3}\\
\to {R_3} = {R_{td2}} - {R_{td}} = 20 - 10 = 10\Omega
\end{array}$