Chapter 33 - Wave Optics - Exercises and Problems - Page 955: 22

$y_1 = 18.2~mm$

We can write an expression for the first secondary maximum: $y_{max,1} = \frac{1.43~\lambda~L}{d} = 26~mm$ We can find the position of the first minimum: $y_1 = \frac{\lambda~L}{d}$ $y_1 = \frac{\lambda~L}{d} = \frac{26~mm}{1.43}$ $y_1 = 18.2~mm$