Given the point $(1, 2)$,find the equation of a line on which it lies. How many such equations are there?

(D) The point $(1, 2)$ must satisfy the linear equation $ax + by = c$. Substituting the coordinates $x = 1$ and $y = 2$,we get $a(1) + b(2) = c$,or $a + 2b = c$.
For example,if we choose $a = 1$ and $b = 1$,then $c = 1 + 2(1) = 3$. Thus,one such equation is $x + y = 3$.
Similarly,if we choose $a = -1$ and $b = 1$,then $c = -1 + 2(1) = 1$. Thus,another equation is $-x + y = 1$.
Since there are infinitely many pairs of $(a, b, c)$ that satisfy the relation $a + 2b = c$,there are infinitely many such linear equations that pass through the point $(1, 2)$.