When solving a system of two equations via substitution, there will always be only one answer.
a. true
b. false?

No false.
Remember the solution(s) to a system of equations is where the graphs all intersect if at all.
For the case of say a system of 2 lines, you can see the different possible outcomes..
  -- If the two lines intersect at some point (x,y), that is one unique solution
  -- If the two lines are parallel to each other, you see there are no intersection points and therefore this system of two parallel lines has no solutuion.
  -- If the two lines overlap, really the same line written as a multiple of the other line, then you see they intersect at all points along the line, here there are infinite solutions.