Slope of the line of 7

Solve the system. -a+4b+2c=-8 3a+b-4c=9 b=-1 a.(0,11) ,-1,2) c.(1,1,-2) d.(2,-1,-1)

Can someone answer this question for me

What would be the reasons for lines 3 and 4? a. addition property; subtraction property b. addition property; solve c. substitution property; subtraction property d. substitution property; prove