Answer
$$\frac{10}{7}$$
Work Step by Step
Since
$$ f'(x)= 2x + 3$$
Then at $x=2, $ $m= f'(2)=7 $. Hence, the tangent line is
\begin{align*}
\frac{y-y_1}{x-x_1}&=m\\
\frac{y-10}{x-2}&=7\\
y&=7x-4
\end{align*}
Since the tangent line intersect with the $x-$ axis at $x=4/7$, then $Q $ has coordinates $( 4/ 7, 0)$, $R $ has coordinates $(2, 0)$, and the subtangent is
$$ 2-\frac{4}{7}=\frac{10}{7}$$
