Answer
$R=\frac{R_1*R_2}{R_2+R_1}$
Work Step by Step
We have $\frac{1}{R}=\frac{1}{R_1}+\frac{1}{R_2}$
Turning the right-hand side into one fraction by multiplying by clever forms of one,
$\frac{1}{R}=\frac{R_2}{R_1*R_2}+\frac{R_1}{R_2*R_1}$
This gives,
$\frac{1}{R}=\frac{R_2+R_1}{R_1*R_2}$
Reciprocating both sides,
$R=\frac{R_1*R_2}{R_2+R_1}$
