Solution

The number of ways of selecting 2 non-defective systems from 4 non-defective systems is given by 

$C_2^4= 6.$

The total number of ways of selecting 2 systems out of 7 systems is given by $C_2^7= 21$

$\stackrel{\text{P( No defective is found )}}{}=\genfrac{}{}{0ex}{}{\text{No. of favorable cases}}{\text{No. of total cases}}$