[tex]\displaystyle\bf\\\Big(\frac{z}{b} -\frac{b}{z}\Big)\cdot\frac{5zb}{z-b} =\frac{z^{2}-b^{2} }{z b} \cdot\frac{5zb}{z-b} =\frac{(z-b)\cdot(z+b) }{z b} \cdot\frac{5zb}{z-b} = \\\\\\=5\cdot(z+b)[/tex]