The common difference is 6.
Now, plug this into the equation for the sum of an arithmetic sequence:
[tex] \frac{n}{2}[2a(1) + (n-1)d] [/tex]
a(1) is 3 (the first number)
n = 36 (the number of terms to sum up to)
d = 6 (common difference)
[tex]18[6 + (35*6)][/tex]
[tex]18[216][/tex]
3888