Microelectronics NMOS amplifier
We have to find out \$g_{m2}, i_d, v_{d1}\$ and \$R_D\$.

Since \$R_{i2} = 1/g_{m2} = 50\Omega,\$  \$g_{m2} = 0.02\mho\$.

Since both transistors are biased at the same point, \$g_{m1} = g_{m2}\$.
$$i_d = g_{m1}v_i = 0.1 mA$$
now,
$$v_{d1} = i_d\times 1/g_{m1}=i_d\times 50\Omega$$

Now the equation in solution manual makes sense. I think you can do the rest your own.