Re: Diode problem
No, the voltage across a forward biased diode is not necessarily 0.7 volts. It is only approximately 0.7 volts when the diode is nearly fully forward biased with sufficient current to do so.
As an easy demonstration, just place a 100 kilohm resistor in place of R1 in figure 1a of your homework. The approximately 100 microamp current will barely even begin to turn on the most sensitive of diodes. In that case Vd would be slightly higher than 0 VDC (the diode is technically forward biased, but only barely.)
Your homework paper says to use the "offset" method and UNDERLINES it. I do not recall a diode offset method and would use an iterative method to solve these. Go back and review the offset method. If Cal Poly uses course numbers like San Jose State did when I was there (they are both CSU schools) a 200 series course is a Master's course. If so and it looks easy, you are likely assuming incorrectly.