Answer
There are a total of 259 arrangement of at most 3 letters form the word NETWORK.
Work Step by Step
Number of arrangements of 1 letter = $C(7,1)$ =7
Number of arrangements of 2 letters = $C(7,1) \times C(6,1)$ = 7$\times$6 = 42
Number of arrangements of 3 letters= $C(7,1) \times C(6,1) \times C(5,1)$ = 7$\times$6$\times$5 = 210
Total = 7+42+210= 259