OK. Without seeing the actual problem  wording, I am going to assume
assume from what you told us that all math  is to be done within the
LinearEquation class. This complicates things a bit, as your two
parameters are not the same units.

Now, onto LinearEquation. This class  solves equations of the form
y=mx+b. You must supply 'm' and 'b', or it does nothing.

I suggest determining which parts of that equation  (y=mx+b) the user
is giving you, then figuring out how to get the parts you need. It can
be done using no math in PhoneBill except one input  transformation to
correct units.

If this weren't an assignment, I would be more forthcoming. However, I
strongly believe in giving someone a map, compass, and directions in
using them rather than airdropping them directly where they want to be.