find z a/2 718=(.58)(.42)(z a/2 / .04) squared

To find za/2 from 0.58*0.42*(za/2 /0.04)² = 718 first divide both sides by 0.58*0.42

(za/2 /0.04)² = 718/(0.58*0.42)

Take the square root of both sides

za/2/0.04 = ±√(718/(0.58*0.42))

Multiply both sides by 0.04

za/2 = ±0.04*√(718/(0.58*0.42))

$${\mathtt{0.04}}{\mathtt{\,\times\,}}{\sqrt{{\frac{{\mathtt{718}}}{\left({\mathtt{0.58}}{\mathtt{\,\times\,}}{\mathtt{0.42}}\right)}}}} = {\mathtt{2.171\: \!618\: \!693\: \!604\: \!038}}$$

So za/2 ≈ ±2.1716