I do not understand why we must add 2 ?

Add 2 is the pattern...

Check for Figure 1
Figure 1, total pupils = 8
By Using my formula = (Figure x 6) +
= (1 x 6) + 2
=8

Check for figure 2, (total pupils = 14)
By Using my formula = (Figure x 6) +
= (2 x 6) + 2
=14

You can check for your answer b) 9 table = figure 9
Check for figure 9,
By Using my formula = (Figure x 6) +
= (9 x 6) + 2
=56

So for Figure 29
Check for figure 29,
By Using my formula = (Figure x 6) +
= (29 x 6) + 2
= 176

Answer for c = number of table = 29

Thanks !