Aloha :)
$$\left.h=\frac{2ab}{a+b}\quad\right|\cdot(a+b)$$$$\left.h(a+b)=2ab\quad\right|\text{links ausrechnen}$$$$\left.ha+hb=2ab\quad\right|-2ab$$$$\left.ha+hb-2ab=0\quad\right|-ha$$$$\left.hb-2ab=-ha\quad\right|\text{links \(b\) ausklammern}$$$$\left.b(h-2a)=-ha\quad\right|:\,(h-2a)$$$$\left.b=\frac{-ha}{h-2a}=\frac{ha}{2a-h}\quad\right.$$