$A$ and $B$ are two points on the $x$-axis and $y$ axis respectively. (a) Write down the coordinates of $A$ and $B$.(b) $P$ is a point on $A B$ such that $A P: P B=$ $3: 1$. Using section formula find the coordinate of point $P$.(c) Find the equation of a line passing through $P$ and perpendicular to $A B$.