If $Z$ is a compressibility factor, vander Waal’s equation at low pressure can be written as

 (P+n²a/V²) *(V-nb) = nRT

For 1 mole, (P+a/V²) *(V-b) = RT

PV = RT + Pb - a/V + ab/V²

At low pressure, terms Pb & ab/V² will be negligible as compared to RT.

So, PV = RT - a/V

$\mathrm{Z}=1-\frac{a}{\mathrm{VRT}}$