we have that
if a||c
then
ma=mc ( if two lines are parallel, then their slopes are equal)
if b||c
then
mb=mc ( if two lines are parallel, then their slopes are equal)
therefore
Remember that
The Transitive Property states that for all real numbers x ,y, and z, if x=y and y=z , then x=z
Applying the transitive property
if ma=mc and mb=mc
then
ma=mb
that means
Line a and Line b are parallel lines because their slopes are equal