(a) $I=nAev_d$ (as derived from the definition of current in terms of drift velocity — see standard derivation): number of electrons crossing area $A$ in time $\Delta t$ is $nAv_d\Delta t$, giving charge $nAv_d\Delta t\, e$, so $I=nAev_d$.

(b) $v_d=\dfrac{eE\tau}{m}=\dfrac{eV\tau}{mL}$ (using $E=V/L$). If $L$ is doubled while $V$ is held constant, $E$ halves, so $v_d$ also HALVES (drift velocity is inversely proportional to the conductor's length, for fixed applied voltage).