Call this point (a,b). Solve line equation for y. Now use the distance formula between the point (1,-3) and (a,b).
Two hints: 1) (a,b) = (a,f(a)) where f(a) is the y-value for x=a
2) The distance formula has a nasty square-root. If you minimize D^2, you also minimize D. So you can drop the square root.
Differentiate and find critical point.