Answer
$19\Omega$
Work Step by Step
We can see that $55\Omega $ and R are connected in parallel
so $\frac{1}{R_{eq}}=\frac{1}{55}+\frac{1}{R}$
$\implies R_{eq}=\frac{55}{55+R}$
Now $R_{total}=12+R_{eq}$
$\implies 12+\frac{55}{55+R}=26$
This simplifies to:
$55R=770+14R$
$41R=770$
$R=19\Omega$
