Hi,
you know that GBW is open loop gain. But since you work with gain of 10 you work with feedback...
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to your problem. You have a very nice picture.
* There is a horizontal line giving DC gain
* then there is an arc. This is where the phase shifts from 0 to 90°
* Then there is a line with constant 20db/decade fall rate. Only here GBW is constant. Also here are three examples that all show a GBW of 10MHz.
To measure at cutoff frequency (-3dB, = 0.707) is possible. Extend the horizontal line a bit to the right, and the constant falling line to the left until they meet. (This should be the -3dB frequency)
Take frequency and gain form the meeting point. (in your case this is A = 10 and frequency = 85kHz, giving a GBW of 850kHz)
Hope this helps.
Klaus