Please see the attached files.
We first look at the OPAMP circuit given and note it is of the inverting type so the magnitude of the gain ( ignore the - sign) is given by the ratio Rf / R1
We are also told this is 20dB which is equivalent to a factor of 100 so we can write
Rf / R1 = 100 so that Rf = 100*R1
Further since we are told R1 = 10kOhm (10^4 Ohm) we can deduce Rf as
Rf = 100 x 10kOhm = 1MOhm = 10^6 Ohm
As an aside also note that in the formula given the factor
(Rf + R1)/R1 = 1 + Rf/R1 =. 1 + 100 =. 101. This will make the maths simpler later ...
This response provides a calculation of the output offset voltage of a inverting OPAMP circuit using the given output offset forum ulna and OPAMP table of characteristics