$7\mathrm{.}24($a$)$ From the plot of yield strength versus $($grain

diameter$){?}^{-\frac{1}{2}}$ for a $70Cu-30Zn$ cartridge brass in

Figure $7\mathrm{.}15,$ determine values for the constants ${}_0$

and $k_y$ in Equation $7\mathrm{.}7.$

$($b$)$ Now predict the yield strength of this

alloy when the average grain diameter is

$2\mathrm{.}0{10}^{-3}mm.$P