Source substitution leads to the following circuit:
The current through 900 ohm experiencing 7.0 V is 7.78 mA; thus the current through R2 must be 2.78 mA. Accordingly its resistance must be 1800 ohm.
Answer for E3.6
(Rhot - Rcold)/Rcold = 4.5 for the stated temperature coefficient and temperature increase. Thus the current surge must be 5.5 times the orerating current for this situation.
Answer for E3.12
I1 = 0.75 A
I2 = 0.25 A
Vx = 2.5 V