Hi, thanks for posting the image, it says a lot.
Math is right, but your measurement is off. It looks like this scope can do this measurement with ease.
If those settings are right, it says its 50nS per division horizontal sweep and 2V p-p.
You do not have it zeroed. Your waveform is 1.5 graticules below the zero mark. Make sure the p-p reach from 0 to 100% on this scope.
Those vertical bars, seem to be movable. The rise time is between the 10% and 90% marks; align one vertical bar at the 10 and 90% mark.
Your trace is either out of focus or its near the limits of its frequency response: its a bit fat or its jittering, thus it looks fat.
I eyeballed that waveform, and with your equation, the rise time gives a frequency response ~ 2 MHz. The roll off near the top of the wave suggests its near the limits of its response.
Now, that is the rise time of the worst case element in your test set up, it could be the probes, the signal generator, or the scope.
Enjoy.
Well, Here a measurement with a signal of 10Vtt, @ 100Khz.
If i do the math: 0.35 / 3.5E-7 = 1 000 000 , so its says my scope is 1Mhz?