Explanation:
we have
B = J + 2
as the number of laps Bill ran.
to get the number of laps Jill ran we need to transform this equation, so that we get
J = ...
or
... = J
J has to be on one side, and the rest on the other (just as it is with B = ...).
so,
B = J + 2
B - 2 = J
or
J = B - 2
hey ! we are finished already !