We know that the sum of all probabilities in a probability mass function (PMF) must equal 1. So, we have:
$$c(x_1 + x_2 + x_3) = 1$$
Since $x_1 = 1$, $x_2 = 2$, and $x_3 = 3$, we can write:
$$c(1 + 2 + 3) = 1$$
$$c(6) = 1$$
Now, we can solve for $c$:
$$c =
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