Apologies in advance, this is probably a really stupid/trivial question (I have no maths experience whatsoever, plus sorry if in the wrong forum!).

My question is simply how does A become equal to the bolded part just under graph (A=sqrt[(2RcD)/Hc] * sqrt[P/(P-D)], given the preceding text? I thought you would just plug the value for Q into A=Q(P-D)/P?

Thanks!