Answer:
Option D is right,.
Step-by-step explanation:
-a+4b+2c=-8 ... i
3a+b-4c=9 ... ii
b=-1 ... iii
Substitute the value of b, in i and ii
-a-4+2c =-8 or -a+2c = -4
and 3a-1-4c = 9 or 3a -4c =8
Now we have two equations in two variables
-a+2c =-4 and 3a-4c =8
a = 2c+4: substitute this in the other equatin.
3(2c+4)-4c =8 Or 2c +12 =8
2c =-4 or c = -2
Substitute in -a+2c =12
-a-2 = -4
a = -2+4 =2
a=2, b=-1 and c =-1 is the solution.
First multiply, so 42 x 39, which equals 1,638, then estimate, round 42 to 40, and 39 to 40, and do 40 x 40 = 1,600 which is close to your answer, so good.
