Hi Kinarfi
Use the general formula for resistors in parallel; Rt = 1/(1/R1+1/R2+1/R3+...). Then transpose terms in the equation in terms of the unknown resistance. Then plug and chug for the solution.
For instance in the simplest terms of one unknown and two known variables, Rt = 1/(1/R1+1/R2). If Rt and R2 are known then solving for the unknown yields the following:
Rt = 1/(1/R1+1/R2) so:
1/Rt = 1/R1+1/R2 so:
1/Rt-1/R2 = 1/R1 so:
R1 = 1/(1/Rt-1/R2)
I know you know this, but sometimes the grey matter just forces one to bang ones head against the wall flattening the head, then when it dawns one flattens the forehead also. LOL! Been there, done that.