\(x= a+\frac{a_{1}-a}{h} \cdot z |-a \quad a=? \\ x-a= \frac{a_{1}-a}{h} · z \quad |: z \\ \frac{x-a}{z}= \frac{a_{1}-a}{h} \\ (x-a) h= z(a_{1}-a) \\ x h-a h =z·a_1 - z·a \quad |-z·{a_1} \\ x h - ah - z·{a_1} = - z·a \quad |+ah \\ x h-z a_1 = a·h - z·a \\ x h-z a_1 = a·(h-z) \quad | :(h-z) \\ a=\frac{x·h-z a_1}{h-z} \)