\(\begin{array}{l}
a)\\
\begin{array}{|c|c|c|}\hline
X&1&2&3&4&5&6\\\hline
P&\dfrac16&\dfrac16&\dfrac16&\dfrac16&\dfrac16&\dfrac16\\\hline
\end{array}\\
\\
\begin{array}{|c|c|c|}\hline
Y&1&2&3&4&5&6\\\hline
P&\dfrac16&\dfrac16&\dfrac16&\dfrac16&\dfrac16&\dfrac16\\\hline
\end{array}\\
b)\\
P(X+Y=3) = P(X=1;Y=2) + P(X = 2;Y=1) = \dfrac16\cdot \dfrac16 + \dfrac16\cdot \dfrac16 = \dfrac{1}{18}
\end{array}\)