Joint PMF to Marginal PMF

The individual PMF from a Joint PMF. $p_X(x)=P(X=x)={\sum }_{y}P(X=x,Y=y)={\sum }_y{p}_{X,Y}(x,y)$

Intuition

Sum all of the PMF values of $y$, and you get the PMF of that specific $x$ Think of it as reading an entire column of values representing $x$