For finding, the sum of first seven numbers which are multiples of 2 as well as of 9.

Take LCM of 2 and 9 which is 18.

So, the series becomes 18, 36, 54, ….

Clearly this series is an AP.

Here, first term, a = 18

common difference, d = 36 – 18 = 18

Now as Sum of first n terms of an AP is

= 7[ 18 + 54]

= 504

Hence the required sum is 504