What property of algebra justifies these statements? A) If x-2 = y-2, then x=y (<-- got this one, addition property of equality)

B) If 5a-2b=7, and b=a-1, then 5a-2(a-1)=7 (still need this one)

(Unfortunately no multiple choice lol)

The john nash mall used 17,4000 kilowatt hours in may, with a peak load of 92 kilowatts. the demand charge is $5.95 per kilowatt, and the energy charge is $0.08 per kilowatt hour. the fuel adjustment charge is $0.017 per kilowatt hour. what is the total electricity cost for may?