You can put this solution on YOUR website!
Get rid of the denominators; multiply thru by (R*R1*R2), resulting in:
R1*R2 = R*R2 + R*R1
Factor out R on the right:
R1*R2 = R(R2 + R1)
Divide both sides by (R2 + R1)
The old "product over sum" formula for two resistors in parallel, right?