Not understanding Level shifting +/-3.5V analog signal for ADC with analog input of 2Vpp and common mode voltage
Do you need DC coupling?  If not then just put a capacitor in series with the input.  If you need DC coupling then you’ll need something more complex.  Meanwhile, you may find that an LF351 gives disappointing bandwidth, depending on what you want to use the scope for.  I probably haven’t quite answered your question but a little more information on what you’re trying to achieve would be helpful.
EDIT:
If you need to accept inputs from -3.5 to +3.5 then you have a couple of options - either use a passive (resistive) divider to shift the level before buffering it with the LF351, or run the LF351 on a split supply (+/-5v).  For simplicity you might prefer the passive approach.  Perhaps try this: use your LF351 as a unity-gain buffer, with the output tied directly back to the inverting input, then connect three resistors to the non-inverting input: one to the 'scope input, one to the +3.3 supply and one to ground.  Work out the values so that you get 1.65v when the scope input is at 0V, 2.65v when the input is at +3.5 and 0.65v when the input is at -3.5v.  The LF351 output can feed directly into the ADC input.