I want to solve for the constant $c$
$\sum_{x_2 = 0}^{2} \sum_{x_1 = 0 }^{2} c(x_1 + x_2) = 1$
So I expanded out the sum
$\sum_{x_2 = 0}^{2} \sum_{x_1 = 0 }^{2} c(x_1 + x_2) = \sum_{x_2 = 0}^{2} \sum_{x_1 = 0 }^{2} cx_1 + cx_2 = \sum_{x_2 = 0}^{2} (\sum_{x_1 = 0 }^{2} cx_1 + \sum_{x_1 = 0 }^{2} cx_2) = \sum_{x_2 = 0}^{2}(3c + 2cx_2) = 3c\sum_{x_2 = 0}^{2}1 + 2c\sum_{x_2 = 0}^{2}x_2 = 3c(2) + 2c(3) = 12c = 1 \iff c = 1/12$
According to Mathematica, the constant should be 1/18.
I cannot figure out what I did wrong for the life of me