a. \(R=R1+R2+R3=5+6+15=26\Omega\)
b. \(I=I1=I2=I3=1A\left(R1ntR2ntR3\right)\)
\(\left\{{}\begin{matrix}U=IR=1.26=26\left(V\right)\\U1=I1.R1=1.5=5\left(V\right)\\U2=I2.R2=1.6=6\left(V\right)\\U3=I3.R3=1.15=15\left(V\right)\end{matrix}\right.\)
c. \(R'=U:I'=26:0,5=52\Omega\)
\(\Rightarrow R_x=R'-\left(R1+R2\right)=52-\left(5+6\right)=41\Omega\)