Can someone please explain

[gre012]

Can someone please explain;

Tow dimensional motion of a body under coulombic attraction to a fixed center.

F = -Kr/r³, that is , it is of magnitude -K/-r and directed radially.
Newton's equation become;

ma = F = Kx/(x² + y²)^3/2

I don't understand how is this equation obtained?

Further, using polar coordinates,
x = r cos(theta)
y = r sin (theta)

How to apply newton's law?