The reciprocal of equivalent capacitance of any number of capacitors joined in series is equal to sum of the reciprocals of individual capacitances.
For n capacitors connected in series, total capacitance would be
$\frac{1}{C_S}=\sum _{i=1}^{i=n}\frac{1}{C_i}$
Or, $\frac{1}{C_S}=\frac{1}{C_1}+\frac{1}{C_2}+\frac{1}{C_3}+\dots$
$\therefore \frac{1}{C_s}=\frac{1}{6}+\frac{1}{6}+\frac{1}{6}=\frac{3}{6}=\frac{1}{2}$
$\therefore C_S=2 pF=2\times {10}^{-12} F$