Are you missing a couple of squared symbols? This problem is more fun if (a-3b) is squared in the first term and (a+3b) is squared in the second. The way you have it written, you need to expand all the terms and get:
2a-6b+9a^2-91b^2+7a+21b = 9a^2 + 9a -81b^2 + 15b
And you can't factor that further that I can see.