Find the coordinates of each point on the graph of the given function where the tangent line is horizontal.

I have this problem:

f(x)=(sqrt x)(x-3).

I used the product rule:

(sqrt x)(x-3)'+(sqrt x)'(x-3)

coming out (sqrt x)+x-3. I know this cannot be right.

What am I doing wrong with this?

The answer is suppose to be (1,-2).