Yes, it should be taken into account
Commenter jdoe said it right: solve for y and leave the rest on the other side.
-x + 3y = 6
3y = 6 + x add x on both sides
3y = x + 6 rearrange to get the x first
y = (x + 6) /3 divide both sides by 3
y = x/3 + 6/3 split the numerator (caution - never split denominators)
y = x/3 + 2 simplify 6/3
Thus the line in slope intercept form of y = mx + b is y = 
I believe the answer would be a.
To answer the question above, use conversion factors and dimensional analysis. In this given, it is easier to use grams as the basis.
a. (2 centrigrams) x (1 gram / 100 centigrams) = 0.02 grams
b. 2 grams
c. (2 hectograms) x (1 gram / 0.01 hectogram) = 200 grams
d. (2 kilograms) x (1 gram / 0.001 kilogram) = 2000 grams
e. (2 milligrams) x (1 gram / 1000 millimeters) = 0.002 grams
Thus, the answer is 2 milligrams.