18. In triangle ABC, m<C=2m<b,m<A=6m<B. what is the measure of <c?

A.5 degrees
B.15 degrees
C.20 degrees
D.30 degrees
E.40 degrees

I do not understand your

m<C=2m<b,m<A=6m<B.
In our general usage of angles and sides, a capital letter usually stands for an angle, a small letter stands for the length of the opposite side to that angle,
there also seems to be a missing equal sign.

If you are saying
angle C = 2m
angle B = m
angle C = 6m

then we know ...
2m + m + 6m = 180
9m = 180
m = 20

so angle C = 2(20) = 40°