Inverting amplifier gain and input resistance
Since the + input of the opamp is grounded, the junction of R1 and R2 will be driven to zero volts, forming a virtual ground, so the input resistance as seen by Vi will simply be the value of R1. For a gain of -10, the output must a generate a negative voltage large enough to drive enough current through R2 to force the R1 R2 junction to zero volts.

If we set R1 to 100k then, and want a gain of -10, the resistance of R2 must be 10 times the resistance of R1 for that to happen, so R2's resistance would be one megohm.