Problem $4\mathrm{.}50$ in Askeland text

$4-50$ The strength of titanium is found to be $65,000$ psi when the grain size is $6\mathrm{.}7{10}^{-4}$ in$.$ and $82,000$ psi when the grain size is $3\mathrm{.}15{10}^{-9}$ in$.$ Determine $($a$)$ the constants in the Hall$-$Petch equation; and $($b$)$ the strength of the titanium when the grain size is reduced to $8\mathrm{.}00{10}^{-6}$ in$..$