12.3 If $a$ and $b$ are distinct prime numbers and $a^{2}-a Q+R=0$ and $b^{2}-b Q+R=0$, find the value of $R$.
$$
a^{2}-5 a+R=0 \text { and } b^{2}-5 b+R=0
$$
$a, b$ are the (prime numbers) roots of $x^{2}-5 x+R=0$
$$
\begin{array}{l}
a+b=5 \ldots \ldots(1), a b=R . \\
a=2, b=3 \Rightarrow R=6
\end{array}
$$
(1), $a b=R$ $\qquad$