Problem: why is this solution undefined
y - 7 2
Solve: --------- = --------
y - 9 y - 9
y - 7 2
--------- = --------
y - 9 y - 9
y - 7 2
(y - 9) * --------- = -------- * (y - 9)
y - 9 y - 9
y - 7 = 2
y - 7 + 7 = 2 + 7
y = 9
Going back to the equation, the denominator, y - 9,
is 9 - 9, which is zero. It is impossible to divide
by zero.