To find the sum of an arithmetic series, we can use the formula:

Sn = (n/2)(a + l)

where Sn is the sum of the series, n is the number of terms, a is the first term, and l is the last term.

In this case, the first term (a) is 6, the last term (l) is 90, and we need to find the number of terms (n).

To find n, we can use the formula for the nth term of an arithmetic series:

l = a + (n-1)d

where d is the common difference.

In this case, the common difference is 12 - 6 = 6.

Substituting the values into the formula:

90 = 6 + (n-1)(6)

90 = 6 + 6n - 6

90 = 6n

n = 90/6

n = 15

Now that we have the number of terms, we can find the sum:

Sn = (n/2)(a + l)

Sn = (15/2)(6 + 90)

Sn = (15/2)(96)

Sn = 720

Therefore, the sum of the series 6 + 12 + 18 + .... + 90 is 720.