Proving that 3 points are collinear

[VanBuren]

Given an acute triangle ABC with altitudes AH and BK. Let N be the midpoint of HK and M - of AB. P is symmetric to M with respect to altitude AH, Q - symmetric to M with respect to altitude BK. Prove that P, N and Q are collinear.

I really don't know even where to start. I see, that circle can be described on ABHK.

 

[stapel]

Given an acute triangle ABC with altitudes AH and BK. Let N be the midpoint of HK and M - of AB. P is symmetric to M with respect to altitude AH, Q - symmetric to M with respect to altitude BK. Prove that P, N and Q are collinear.

I really don't know even where to start. I see, that circle can be described on ABHK.

I don't understand the various "minus" signs in this exercise. For instance, what does your book mean by "Let N be the midpoint of HK and M - of AB"? What is "Q - symmetric to M"? And so forth?

Is there a picture that goes with this exercise?