3dB frequency of inverting amplifier
Get some log-log graph paper and draw an X axis up to 1 MHz
Draw a Y axis that covers the region from unity gain to 100 or 1000
Draw a line from (1 MHz, 1) with negative slope as per purple below
draw a line a horizontal line at gain 20 (blue) until it intersects the purple line
Project that intersect down to the X axis





Read the frequency on the X axis.